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ELEMENTS

OF

STATIC ELECTRICITY

FULL DESCRIPTION OF THE HOLTZ AND TOPLER MACHINES

AND THEIR MODE OF OPERATING.

/By PHILIP ATKINSON, A.M., Ph.D.

NEW YORK:

W. J. JOHNSTON, PUBLISHER,168-177 POTTER BUILDING.

1887.

M

Copyright, 1886,

By W. J. Johnston.

>LS

INTRODUCTION.

In this treatise the principles of electricity are

presented untrammeled, as far as possible, by mathe-

matical formulas, so as to meet the requirements of a

large class who have not the time or opportunity to

master the intricacies of formulae, which are usually so

perplexing to all but expert mathematicians.

This class includes those whose knowledge of

electricity is limited to the practical details of teleg-

raphy, telephony, and electric lighting; also those

among the liberally educated, who desire to review

electric science in the light of recent investigation;

and those who wish to study its elementary principles,

preparatory to a more extended course, which shall

embrace all the details of electric measurement and

electric engineering.

The original plan included dynamic as well as static

electricity, embracing its practical application to the

arts; but it was subsequently thought best to confine

the present work to static electricity alone, to meet

the wants of those who are less familiar with its prin-

ciples than with those of dynamic electricity, and to

IV 1XTR0D UCTIOX.

reserve the consideration of the latter for a separate

volume.

Care has been taken to avoid the introduction of

new matter before the student was prepared for it

;

hence it was thought best that there should be a

thorough examination of elementary principles before

introducing complicated apparatus, the construction

and operation of which depends on those principles.

The theory assumed is, that electricity is one of

the forms in which energy manifests itself; that its

nature is not changed by the means emplo}red to

generate it, and that the various terms, positive,

negative, static, dynamic, express certain conditions and

relations in which this manifestation occurs, and

not different kinds of electricity.

The author takes pleasure in acknowledging his

obligations to Elisha Gray for the use of tables, giving

the results of observations on earth currents, made

under his direction on the Postal Telegraph line; also

to Ferguson, Gordon, Silvanus P. Thompson, Noad

and Deschanel, from whose excellent works valuable

assistance has been obtained, though he has felt

compelled to dissent from some of their views.

The views here expressed are the result of many

years' experience in the class room, the lecture room,

and the laboratory, and were adopted only after the

most rigid test of actual and oft repeated experiment.

And some of the more important apparatus described

INTRODUCTION. V

is of the author's own manufacture, constructed in

strict accordance with electric principles, verified by

his own experiments.

While humbly following the great pioneers in

electric science, who have hewed waymarks on the

rocks, the author will rest content if he has left some

foot-prints on the sands, which may serve to guide the

wayfarer till obliterated by the coming waves of

progress.

The impartial criticism of teachers and electricians

is especially requested, that faults and errors may be

corrected in future editions.

PHILIP ATKINSON.

Chicago, June, 1886.

CONTENTS.

CHAPTER I.

Page

Attraction and Repulsion, 1

Conductors and Non-Conductors, ... 4

Quantity and Intensity, 6

Static Electricity Defined, 8

CHAPTER IT.

Electric Potential, 10

CHAPTER HI.

The Nature of Electricity, 23

CHAPTER IV.

Induction, 43

CHAPTER V.

Electric Distribution and Condensation, . . 55

CHAPTER VI.

Accumulators, 72

Vlll CONTENTS.

CHAPTER VII.

Electric Generators.—The Electrophorus and Frictional Machine, 92

CHAPTER VIII.

Electric Generators.—The Holtz and Topler Machines, . . . los

CHAPTER IX.

Experiments with the Topler Machine, . . . 125

CHAPTER X.

Electric Transmission in Vacua, .... 14(5

CHAPTER XI.

Electrometers, . 155

CHAPTER XII.

The Electricity of the Earth and Atmosphere.—Potential and Earth Currents, . . . 175

CHAPTER XIII.

The Electricity of the Earth and Atmosphere.—The Aurora, 190

CHAPTER XIV.

The Electricity of the Earth and Atmosphere.—

Lightning and Thunder, ..... 207

ELEMENTSOF

STATIC ELECTRICITY.

CHAPTER

Attraction and Repulsion— Conductors andNon-Conductors—Quantity and Intensity—Static Electricity Defined.

Attraction and Repulsion.—Amber, called in

Greek tjtextQov, was known to the ancients to acquire,

when rubbed, the power of attracting light bodies

;

hence this property, now known to belong to all sub-

stances, has received the name of electricity. Theearliest conception of electricity, then, was that of

force and the latest discoveries sustain this view.

Electricity may be generated by various simplemethods, as follows:— Let a spoon be balanced on theedge of a cup. and an ebonite (hard rubber) knife-handle, rubbed on a woolen or silk fabric, be heldnear it. and the spoon will be attracted. Substitute forthe knife-handle a stick of sealing-wax. a lamp-chim-ney, or a paraffin wax-candle, rubbed in the same way.and the spoon will be attracted by each of them.

These different substances may be multiplied, anddifferent rubbers used, but it will be found that the

Z ELEMENTS OF STATIC ELECTRICITY.

attractive force, though variable in intensity, is com-

mon to all.

The balanced rod, represented in Fig. 1, will be

found more convenient for these experiments than the

balanced spoon. It consists of a round wooden rod,

about twenty inches long, and half an inch in diameter,

with the ends rounded or terminating in balls. It is

pivoted at the center on a point, and may be mounted

on a stand, or on a bottle with a pin through the cork,

and made to revolve rapidly by the force of attrac-

tion, following any of the electrified bodies already

mentioned when held near it, as represented.

Fig. 1—The Balanced Rod.

A more sensitiveinstrument for investigations of this

class is represented in Fig. 2, and known as the pith-

ball electroscope; the name electroscope being derived

from the Greek axonem, to see, i]lr/.rnor, electricity. It

is constructed as follows.: A small brass rod, bent at

right angles, has its short arm inserted into an ebonite

stem attached to a wooden base, giving the instrument

a vertical height of about Hi inches. The horizontal

arm is about 8 inches long, and terminates in a small

brass ball. From this arm two pith balls, each about

half an inch in diameter, are suspended by silk threads.

ATTRACTION AND REPULSION.

Let the pith balls be separated at the points of sus-

pension, so that when they hang vertically a consider-

able space shall intervene between them, and let a

stick of sealing-wax, previously electrified by friction,

be brought near one of them ; the ball will be attracted

to the wax, and, after a momentary contact, repelled.

Follow it with the wax, and it continues to recede as if

pushed back by some invisible barrier.

Now let the other pith

ball be moved near this

one, and they will be

attracted to each other,

and, after contact, re-

pelled : the lines of sus-

pension showing diver-

gence in each direction

as represented.

Let the electrified waxbe again brought near,

and each ball is repelled

by it, so that when it isFig

'2~Tbe Pitll"Ba11 E1^roscoPe.

placed between them, they are driven further apart;

but let any non-electrified body be brought near andthey are attracted to it.

If each of the balls be separately electrified by the

wax, and they are then brought near each other, they

will show mutual repulsion without previous attraction.

From this series of phenomena we learn, first, that

electrified bodies not only attract non-electrified bodies,

as already shown, but communicate electricity to themby contact ; and, secondly, that bodies electrified, either

by each other or from the same source, show mutualrepulsion.

4 ELEMENTS OF STATIC ELECTRICITY.

The first fact was shown when the pith ball, after

contact with the wax, attracted and electrified the

other pith ball; and the second fact by the repulsion

of the pith ball from the wax after contact; then of

the two pith balls from each other and from the wax,

after contact ; and finally by the mutual repulsion of the

balls, without previous attraction, after being separately

electrified by the wax.

This series of phenomena may be produced by using

a glass or ebonite rod, or anj^ of the substances already

mentioned, as well as by the sealing-wax; showing that

repulsion as well as attraction is a property common to

all electrified bodies.

Conductors and Xox-Coxductors.—Pursuing our

investigation, new properties are developed. It is

found that while certain substances, as glass, ebonite,

and sealing-wax, show electric qualities, others, as brass,

iron, and copper, apparently do not show such qualities.

This led to the old division of all substances into

electrics, a term applied to the former, and non-electrics,

applied to the latter.

But more thorough investigation has proved that

electricity may be generated by friction on the brass,

iron, and copper, as well as on the glass, ebonite, and

sealing-wax ; but that, when generated on bodies of the

former class, it is instantly distributed over the entire

body, and escapes to the earth unnoticed, if the body

be held in the hand, while, when generated on bodies

of the latter class, it is not so distributed, and does not

pass off in this way; bodies of the former class allowing

free electric movement, over the surface or through the

mass, while those of the latter class resist such move-

ment.

CONDUCTORS AND NON-CONDUCTORS, 5

To make this evident, let a short brass rod, of about

quarter inch diameter, terminating in a ball, be fitted to

an ebonite handle, as

represented by Fig. 3.

Let the brass be rubbed

briskly on woolen, silk,Fi- Z~ThQ Insulated Metal Rod.

or India rubber, holding the instrument by the handle,

audit will attract and repel the pith balls in the same

way as the other electrified substances already used.

Copper, iron, or any other metal may be substituted

for the brass with the same result.

Repeat the experiment, allowing the metal to touch

the hand, and the electric qualities disappear. This

shows that in the first experiment the electricity wasretained, because it could not pass through the ebonite

handle ; while, in the second, it passed off through the

hand.

The results obtained by experiments of this kind led

to the abandonment of the doctrine of electrics andnon-electrics, and the classification of all bodies as con-

ductors or non-conductors.

Experiment proves that all substances conduct elec-

tricity, and that they all offer a certain amount of

resistance to its passage. But it is found that therelative proportions of conductivity and resistance varygreatly in different substances. In some the conduct-ivity is largely in excess, and they are called conductors ;

in others the resistance is largely in excess, and theyare called non-conductors. Between these extremesthere are all degrees of variation; so that in some sub-stances the two properties are almost equally balanced.Hence, since no exact rules can be given, we distinguishthe two classes by saying that a CONDUCTOB i* any suh-

b ELEMENTS OF STATIC ELECTRICITY.

stance of such low resistance that it can be used practi-

cally for the transfer 'of electricity ; and a NON-CON-DUCTOR is any substance of such high resistance that it

can be used practically to prevent such transfer.

List of Conductors and Non-Conductors.—Theprincipal conductors are the metals, silver and copper

being the best. Among the partial conductors are the

different varieties of carbon, including coal, charcoal,

and graphite ; the acids, saline solutions, water, vegeta-

bles, and animals.

The principal non-conductors are caoutchouc, gutta-

percha, sulphur, and their compound, known as hard

rubber, vulcanite, or ebonite ; dry air, paraffin, shellac,

amber, resin, glass when free from metallic substances,

mica, silk, fur, wool, hair, feathers, bisulphide of carbon,

petroleum, and oil of turpentine.

Among the partial non-conductors are porcelain,

baked wood, paper, and leather.

Insulator Defixed.—When a non-conductor is

used in connection with a conductor to confine elec-

tricity within certain limits, it is called an insulator;

and the conductor on which the electricity is confined,

or to be confined, is said to be insulated ; as a metal

placed on a glass or ebonite support, a copper wire

wrapped with silk or wool.

Quantity and Intensity.—Electric quantity and

intensity are similar to the quantity and intensity found

in other more familiar forms of energy. The intensity

of any form of energy, other things being equal, is in-

versely proportional to the mass of the body in which it

is developed. A few strokes of a hammer on a small

piece of iron placed on an anvil will raise its temper-

ature to a burning heat; while the same number of

QUANTITY AND INTENSITY. 7

strokes on a large mass of iron will produce but very-

slight change of temperature. The quantity of muscular

energy expended is the same in each case, but the inten-

sity of heat energy produced varies inversely as the

mass.

The intensity varies also as the resistance. A small

piece of wood held in the flame of a lamp is quickly

ignited at the end in the flame ; while the end held in

the hand shows no perceptible change of temperature.

But a brass rod of the same size, similarly held for the

same length of time, becomes too hot for the hand long

before the end in the flame is hot enough to ignite

wood.

In the wood, the intensity rises rapidly at the end

held in the flame, because the resistance prevents dis-

tribution of heat through the mass. But the low

resistance of the brass permits the rapid distribution

of the same quantity of heat through its mass ; so that the

intensity at the end in the flame is much less than that

of the wood.

In kindling a fire of anthracite coal, when the pro-

portion of coal is too great for the kindling-wood, the

heat generated by the consumption of the wood fails to

ignite the coal, because such coal being a comparatively

good conductor of heat, the amount is rapidly distrib-

uted through the mass, and hence the intensity at anypoint is insufficient to produce ignition. But if the pro-

portion of coal be sufficiently reduced, the consumptionof the same amount of wood will produce ignition.

The quantity of heat imparted to 1 lie coal is the same in

each case, but its intensity is greater in the latter case.

In electric experiments there is a great difference

noticeable in the amount of work required to produce


the same electric intensity on different bodies. Sealing-

wax and ebonite, for instance, are quickly electrified,

while brass is electrified slowly. The reason is analo-

gous to that in the illustrations just given : the brass

being a good electric conductor, the electricity is in-

stantly distributed equally over every part of its sur-

face, and hence the quantity at any point being small,

the intensity is low. But the sealing-wax and ebonite

being good non-conductors, the same quantity of elec-

tricity is concentrated on those parts of the surface

brought into immediate contact with the rubber, instead

of being equally distributed over the entire surface;

and hence the intensity at those points is proportion-

ately increased.

It will be shown hereafter that in static electricity

the electrification is on the surface. Hence, in this

case, electric intensity means quantity in proportion to

surface, whether it be the entire surface, as on a con-

ductor, or only those parts to which the electrification

is confined, as on a non-conductor.

It must also be understood, as will be shown more

fully hereafter, that the term intensity is as applicable

to a diminution of electric energy at a given point as

to an increase ; in the same sense as we speak of intense

cold, as well as of intense heat.

Static Electricity Defined.—The terms used to

distinguish different classes of electric phenomena, as

frictionah static, galvanic, chemical, magneto, thermo,

take their origin from the different methods by which

electricity is generated, and the various conditions under

which its phenomena have been observed, and should

not be understood as referring to any difference in

the nature of the electricity produced.

STATIC ELECTRICITY DEFINED. 9

The term frictional has been used to designate that

class of phenomena now under consideration, since

friction is one of the principal agencies by which the

electricity is generated. But it seems more appropriate

to use a term embracing, not merely one agency by

which the electricity is generated, bat also the various

phenomena produced, and distinguishing these phenom-

ena from those pertaining to electricity generated by

other methods. And since these phenomena refer

chiefly to electricity when stationary, the term static,

from the Latin sto, to stand, has been adopted, to dis-

tinguish electricity observed under these conditions

from electricity observed chiefly in a state of motion.

CHAPTER II.

Electric Potential .

Potential.—Potential, in the physical sense, is the

power to accomplish work. It derives its specific namefrom the nature of the work, as gravity potential, heat

potential, electric potential.

A pound weight raised to the height of ten feet has

acquired ten foot-pounds of gravity potential, and has

the power, if allowed to descend to the same level, of

accomplishing ten foot-pounds of work, either in rais-

ing another weight, or setting machinery in motion by

which work may be accomplished.

A mass of metal whose temperature has been raised

from zero to one thousand degrees, has acquired one

thousand degrees of heat potential, and can accomplish

work to that amount in cooling to zero, either by heat-

ing another mass, or generating steam by which machin-

ery can be put in motion and work accomplished.

We have seen that bodies, when electrified, acquire

the power to attract or repel other bodies. This power

is called electric potential.

Suppose that the electric energy of the sealing-wax

in attracting the balanced rod, represented in Fig. 1,

Chapter I., w^ere just sufficient, if expended without

loss, to move the rod one foot ; and, in doing so, to

overcome a resistance from inertia and friction repre-

sented by two ounces (one-eighth of a pound ) ; the

ELECTRIC POTENTIAL. 11

electric potential of the sealing-wax would equal one-

eighth of a foot-pound.

If only half this energy were required to overcome

inertia and friction, the other half might be expended in

lifting to a height of one foot an ounce weight attached

to a thread fastened to the end of the rod, and passing

over a pulle}'. In which case the work accomplished

by this half would be represented by one-sixteenth of

a foot-pound. Or the weight might be raised, or other

work to the same amount accomplished, by putting in

motion light machinery connected with the rod by gear-

ing at its center ; the added friction being included in

the ounce representing friction and inertia.

Impulsion would evidently produce the same results

in this case as attraction.

To distinguish between electricity and electric poten-

tial, we must bear in mind that electricity represents

the energy itself, while potential represents certain rela-

tions between this energy and matter. Hence we derive

the following definition :

Electric potential is the power which a body possesses to

accomplish work by virtue of its electricity.

Difference of Potential.—To accomplish workin tliis way there must first be a difference of po-

tential.

The descending weight could not raise the other

weight unless there was a difference of level betweenthem. Tin,' heated metal could not heat a similar massunless there was a difference of temperature betweenthem. Neither could the electrified sealing-wax attractthe rod unless there was a difference of electric energybetween them. And these phrases, difference of level.

difference of temperature, difference of electric energy,


are simply different forms of expression for difference of

potential.

To produce this difference work must first be ex-

pended, and this work is the measure of the potential

acquired.

The lifting of the pound weight ten feet against the

force of gravity gave it the ten foot-pounds of gravity

potential. The work of heating the metal, whether

represented by combustion, by friction, or by concus-

sion, gave it the one thousand degrees of heat potential.

And the rubbing of the sealing-wax gave it the one-

sixteenth of a foot-pound of electric potential.

As there is ordinarily no practical difference of elec-

tric potential between different points on the earth,

within a limited area, its potential is considered zero,

and taken as the base of all measurements of electric

potential.

The qualification of this statement, as above, becomes

necessary, since there are often great differences of po-

tential over widely separated areas.

Positive and Negative.—Bodies whose potential

is higher than that of the earth are said to have positive

potential, while those whose potential is lower are said

to have negative.

The potential of bodies is also considered positive or

negative with reference to each other. If a body has

a higher potential than the earth, but lower than that

of another body, it is said to be positive with reference

to the earth, but negative with reference to the other

body. In like manner a body may have negative

potential with reference to the earth, but positive with

reference to another body of lower potential.

Hence, positive and negative are merely convenient rela-


tive terms to designate different degrees of potential and

not different kinds of electricity.

The sign ( + ) is used to denote positive potential, and

(—) to denote negative potential. *

The earth's potential, then, is the electric zero, just

as the freezing point is the zero of temperature in the

centigrade thermometer, and all uninsulated bodies are

said to be connected ivitli the earth, and to have zero

potential when not under special influence from insu-

lated, electrified bodies in their vicinity.

When the electric potential of a body is changed

from zero by an increase of its electricity, it is said to

be positively electrified ; and when its potential is

changed from zero by a decrease, it is said to be nega-

tively electrified.

Electric Movement.—When a difference of electric

potential exists between different bodies, or different

parts of the same body, there is a constant tendency to

equalization.

A state of equilibrium seems to be the natural condi-

tion of bodies, and to produce difference of potential

requires, as we have seen, the exercise of force in the

performance of work, by which this equilibrium is dis-

turbed.

We find in other forms of energy, as gravity and heat,

the same tendency to equilibrium, requiring the exerciseof force to overcome it, as in the illustrations alreadygiven.

The restoration of equilibrium is always effected bya transfer of energy from the body having the greaterto the one having the less energy; that is, from higherto lower potential.

In the case of gravity this transfer of energy carries


the body with it, as in the descent of a weight or the

movement of water from a higher to a lower level.

But in the case of heat and electricity, the energy maymove while the body remains stationary ; and it may be

transferred from one body to another, or from one part

to another of the same body. Thus the mass of metal,

in the illustration given, transfers its heat energy to

another mass ; and in like manner, when a metal rod is

heated at one end, the heat moves to the cold end.

Gravity apparently can move only by carrying the

body with it, while heat moves through the body with-

out producing change of position in its mass, like gravity.

A hot body transfers its heat to a cold one in its

vicinity, but does not attract it ; while gravity produces

mutual attraction between all bodies, but is not trans-

ferred like heat from one body to another.

But in electrified bodies we have both kinds of move-

ment. Like heat, electricity can move from one body

to another, or from one part to another of the same

bod}' ; and, like gravity, it can cany the body with it.

Hence we must distinguish between the movement of

electricity and the movement of the electrified body.

Electric movement, like heat movement, is from higher

to lower potential. If one part of a conductor be elec-

trified, the electricity instantly distributes itself over

every part. If two insulated bodies, free to move, are

placed in each other's vicinity, like the pith balls of the

electroscope, the same tendency to equilibrium is shown

by their mutual attraction.

Though only one ball be electrified, yet it is evident

that their movement toward each other must be mutual,

and in proportion to their mass, since action and reac-

tion are equal : so that while the movement of the elec-


tricity is from the electrified to the non-electrified ball,

that is, from higher to lower potential, the movement

of the balls is mutual.

It will also be noticed that the movement of the non-

electrified ball is opposite to that of the electricity.

Hence, while electricity moves from higher to lower

potential, bodies under its influence may move in -either

direction.

We have seen that when the two balls come into

contact there is a transfer of electricity from the elec-

trified to the non-electrified ball ; equilibrium is estab-

lished, and mutual repulsion follows, not only between

the balls, but also between them and the electrified

sealing-wax.

So long as a difference of potential exists there is

mutual attraction; but when equilibrium is established

there is mutual repulsion. The same results may be

produced by numerous similar experiments, in whichdifferent substances and different methods may be

employed. Hence we deduce the following important

principle

:

•

Electrified bodies at different potentials attract, tvhile

those at the same potential repel each other.

There can be no repulsion unless there is a difference

of potential between the electrified bodies and their

surroundings. For if the surrounding bodies were at

the same potential as the electrified bodies, the repul-

sion would be neutralized by their reaction. Hencebodies at zero potential can show no repulsion. But in

all cases of electrification there is a difference of poten-tial created in the body, either above or below the origi-

nal zero.

Indeed, attraction may account for the apparent

16 ELEMEXTS OF STATIC ELECTRICITY.

mutual repulsion of bodies at the same potential, since

this difference of potential between the electrified bod-

ies and their surroundings must produce attraction and

tend to separate them.

But such outward attraction would not disprove the

existence of repulsion, though it might account for

some of its phenomena.

The Gold Leaf Electroscope —As our investi-

gations now require a more sensitive instrument than

any which has yet been described, we here introduce

the gold leaf electroscope.

-3

1

Fig. 4—Gol-1 Leaf Electroscopes.

The style represented at A, Fig. 4, is convenient, and

easily constructed. It consists of a half-gallon tincture

bottle, fitted with an ebonite stopper, through the center

of which passes a small brass rod about five inches long,

which terminates about three-fourths of an inch above

the stopper in a brass disc about two inches in diame-

ter, having a round rim about three-sixteenths of an


inch in diameter projecting from its lower surface, as

shown in the enlarged section at D.

To the lower end of the rod is attached a thin cross-

bar, about five-eighths of an inch long, which will pass

easily through the neck of the bottle. And from this

cross-bar are suspended two strips of imitation gold leaf,

each five-eighths of an inch wide by 2 J- inches long. Asmall hole is drilled near the edge of the disc for con-

venience in attaching wires.

The leaves in this instrument lie close together, and,

consequently, must always be electrified at the same

potential; but in some experiments it is desirable to

electrify them separately, and for this purpose a bottle

with a wide neck is used, which will admit an ebonite

stopper through which two rods can be inserted about

an inch apart, and from the cross-bar of each a single

leaf is suspended, the surfaces being parallel to each

other. This style is represented at B, Fig. -i. The rods

can terminate above in balls, or be bent outward andterminate in discs.

Electroscopes may be constructed with thin metal

discs, attached to the glass opposite the leaves ; strips

of the same material extending down and connecting

with the earth. Brass rods surmounted with balls are

often used in the same way, as represented at C\ Fig. 4

;

in which case a glass shade resting on a wooden base is

more convenient than the bottle form.

The object in either case is to have conductors at zero

potential near the leaves, which renders them moresensitive, and discharges them in case of too great

divergence; thus preventing their adhesion to the glass,

which is often troublesome. Annoyance from the latter

cause is also obviated by using a bottle of globular form,


the sides of which are too remote to be touched by the

leaves.

A brass cap, covering the glass above, as shown at (7,

is also used to screen the leaves from external electric

influence, and wire screens are likewise used for the

same purpose.

The use of the bottle, or glass shade, is to protect the

leaves from currents of air which would destroy them.

And the ebonite stopper is for better insulation, since

the glass generally used for bottles and shades is of

inferior insulating quality. The disc, or ball, and con-

necting-rod are for convenience in electrifjdng the

leaves, which are the efficient part of the instrument.

The following experiment will illustrate its use:

—

Let the electrified sealing-wax touch the disc of electro-

scope A ; electricity is instantly transferred to the disc,

rod, and leaves, which are all good conductors, and the

leaves, being free to move, and at the same potential,

repel each other, and diverge.

If the disc now be touched with the finger, the elec-

tricity escapes to the earth, and the leaves, being reduced

to zero, converge.

The sensitiveness of this instrument is so great that a

chip of dry wood, less than a grain in weight, electrified

in cutting, and dropped on the disc, produces divergence

of the leaves. A wooden pen-holder, electrified by strik-

ing it lightly on the table, produces the same effect.

Hence, care must be observed to prevent the leaves

from being torn by sudden, spasmodic movements, which

are liable to occur when experimenting with highly elec-

trified bodies in their vicinity.

Mutual Effects of Friction.—Thus far we have

considered only the effect produced on the sealing-wax,


glass, or other substance electrified by friction, without

reference to the effect on the substance by which it was

rubbed. But since action and reaction are equal, it is

evident that these two effects must, in some way, equal

each other ; that electricity, or its equivalent in some

other form of energy, must be produced on the rubber

as well as on the substance rubbed.

To test this, let a piece of flannel, after being used to

rub a stick of sealing-wax, touch the disc of electroscope

A, Fig. 4, and the leaves will instantly diverge, showing

that the flannel has been electrified.

Substitute silk, fur, or any other substance used as a

rubber, and the same result will follow. Let the various

substances rubbed be also tested, and it will be found

that electrification has been produced on both rubber

and substance rubbed, at the same time, by the sameprocess.

Now let a rubber, about the same size as the sealing-

wax, be prepared, by wrapping a strip of wood in flannel

and insulating one end with a piece of india-rubber

tube.

Holding this rubber by the insulated end, let the

sealing-wax be rubbed with it ; and, keeping both still

in contact, lay them carefully on the disc of the electro-

scope, so that both shall touch it at the same instant,

and no divergence of the leaves will occur. Now lift

off the sealing-wax and they instantly diverge; replace

it and they converge. Lift off the rubber and they

diverge, replace it and they converge again.

Let the experiment be made with any other two sub-

stances used to generate electricity by friction, as silk

and glass, ebonite and fur, and similar results will beobtained.


It will also be noticed that the approach of either

electrified body while the other lies on the disc causes

the leaves to converge, while its withdrawal produces

divergence.

There is often a slight divergence of the leaves when

both bodies are in contact on the disc, due to the diffi-

culty of producing perfect adjustment of contact, and

also to the fact that the electric condition of one body

may change more rapidly than that of the other, from

imperfect insulation or other cause.

The amount of divergence is also liable to vary, the

removal of one body producing greater divergence than

the removal of the other. This difference is also easily

accounted for by difference of mass, of conductivity, or

other cause.

Hence we deduce the following rule : When electricity

is generated on two bodies by their mutual friction, the elec-

tricity of each is neutralized by the presence of the other.

The effect of the mutual friction of the two bodies is

to create a difference of potential by the transfer of

electric energy from one to the other. As one gains

what the other loses, the amount of energy on the two

is not changed so long as they remain in contact, and

hence the potential of the electroscope is not disturbed.

But let one of the bodies be removed ; suppose it to

be the one to which energy has been transferred, the

potential of the remaining body being negative, there is

instantly a transfer of energy to it from the disc and

leaves, which thus become negative also.

The leaves, being both at the same potential, diverge

by mutual repulsion ; and that potential being less than

zero, the divergence is increased by attraction from the

higher potential of the glass and surrounding objects.


Replacing this body, let the one from which energy

has been transferred be removed ; the potential of the

remaining body being positive, there is instantly a

transfer of energy from it to the disc and leaves, making

them positive also. Hence the leaves diverge as before,

from mutual repulsion, and the divergence is increased

by attraction from the loiver potential of the glass and

surrounding objects.

From this it will be seen that the effect on the electro-

scope is the same whether the potential of the electrified

body be positive or negative. In either case there is

mutual repulsion between the leaves, from their being

at the same potential ; and mutual attraction between

them and surrounding objects, caused by difference of

potential.

The indications of the electroscope furnish no meansof distinguishing between positive and negative poten-

tial, being the same for both. And as this is true of

most of the phenomena pertaining to these two states,

it is difficult, in static electricity especially, to determine

which phenomena are positive and which negative.

There is no such well-marked distinction betweenthem as between the positive and negative states knownas heat and cold; neither can we observe electric move-ment as we can heat movement; since heat movesslowly, while electricity moves with inconceivable ra-

pidity.

But if we can show cause for an accumulation of elec-

tric energy at one point and Tor its absence at another,

and show effects following such difference of energy, wethen have proof of the positive and negative potential

of the different points, which may be accepted as

reliable.


Such proof will be furnished hereafter, and the further

consideration of this question must be deferred till the

examination of other phenomena shall enable the stu-

dent to comprehend such proof.

Charge Defined.—The term charge is used to ex-

press the condition of an electrified body when its poten-

tial is above or below zero. If its potential has been

raised above zero by receiving electricity, it is said to be

positively charged ; but if its potential has been reduced

below zero by the removal of electricity, it is said to be

negatively charged.

Hence we speak of a high negative charge in the same

sense as we speak of intense cold, meaning an intensity

of the negative condition caused by the absence of heat.

CHAPTER III.

The Nature of Electricity.

The Conservation of Energy.—A clear under-

standing of that great doctrine of modern science,

known as the conservation of energy, lies at the founda-

tion of a correct knowledge of electricity and electric

phenomena. Hence a brief examination of its prin-

ciples will not be out of place here.

Energy is a universal property of matter. It is the

principle of life and movement in matter in distinc-

tion from matter itself, inseparably connected with

matter and yet distinct from it : heat as distinct from

the heated body ; electricity as distinct from the elec-

trified body ; life as distinct from the living body.

Like matter, it manifests itself in various forms,

as gravity, cohesion, chemical affinity, light, heat, elec-

tricity. Like matter, its quantity in the universe is

fixed and definite, and cannot be increased or dimin-

ished. And hence, like matter, it is indestructible.

It maybe transmuted- from one form into another,

but in the transmutation there is no loss. One form

may re-appear in many forms, or the many be reduced

to the one.

In our experiments, muscular energy lias been ex-

pended to produce electric energy; but the energy pro-

duced must equal that which produced it, if the doc-

trine of the conservation of energy is true. And since


it is evident that only a very small part of the mus-

cular energy expended would be required to movethe pith balls, the balanced rod, or the gold leaves, the

remainder must be accounted for.

This is easily done when we consider, first, that

the electric energy was equally divided between the

rubber and the substance rubbed; secondly, that only

a small part of the electric energy was used : that the

electricit}^ generated was sufficient for the performance

of the same work many times in succession, either

with the rubber or substance rubbed; and that a

number of pith balls, placed on all sides of the

electrified body, might have been subjected to its

influence. Thirdly, we must consider the amount of

electricity lost from contact with the surrounding air:

and, lastly, that the amount of heat energy gener-

ated by the friction was probably equal to the electric

energy.

If the expended energy had been produced by a

descending weight, which should cause a glass or

ebonite cylinder to revolve in contact with a rubber,

and the sum total of the heat and electricity had

been recovered in the form of work which could be es-

timated, it would be found so nearly equal to the

number of foot-pounds expended by the descending

weight, that whatever difference existed could easily

be accounted for by the friction of the machinery and

other causes.

Experiments of this kind have been actually per-

formed, and the results verify the above conclusion.

Similar experiments have also been made with other

forms of energy, and like results obtained; so that

the principles of the conservation of energy are now

THE NATURE OF ELECTRICITY. 25

well established, and universally recognized in all

practical work.

Another illustration may render the subject more

clear. A pound weight raised to a height of 20 feet

has acquired 20 foot-pounds of energy ; and, in de-

scending to its former level, can accomplish 20 foot-

pounds of work; as in raising to the same height

another weight of nearly equal mass. But if stopped

in its descent at a level of 10 feet, it has expended

only 10 foot-pounds of energy, and has still a reserve of

10 more.

It is evident) that to raise the weight in the first place

required the expenditure of 20 foot-pounds of energy

;

and that though this energy was consumed in the pro-

>s, and had disappeared, it was not lost, but merely

stored up, ready to be expended, either at once, bythe weight descending the entire distance, or in detail,

as when stopped half way or at any other point. If it

had descended but one foot, it would still have a re-

serve of 19 foot-pounds of energy.

It will be noticed that the second weight, raised bythe descent of the first, is required to be of less

magnitude, since part of the energy must be expendedin overcoming friction and inertia. For* if it were of

equal magnitude, the force expended would exceed the

force stored up; Bince it must perform not only the

same work-, but the added amount for friction and

inertia; in which case it would be possible to create

force, and the doctrine of the conservation of energywould cease to be true.

It is immaterial whether the descent of the one poundraises a small weight to the height of 20 feet, or a

large weight to the height of one foot; which it can be


made to do by a system of ropes and pulleys. The20 foot-pounds of energy expended must exactly

equal the 20 foot-pounds stored up; and the height

through which the large weight is raised is to that

through which the small weight descends in the in-

verse ratio of the mass of each.

In all electric work, of whatever nature, the sameprinciple will be found to hold true

; gravity potential

in this case representing electric potential in electric

work. Mechanical work may re-appear as electric work,

or electric as mechanical work; the energy produced

being always, in some form, equal to the energy

expended.

Heat, Light, and Electricity Compared.—Theweight of evidence goes to show that electricity, like

heat and light, belongs to that kind of energy knownas molecular ; and whatever is known as to one kind of

energy may, by analogy, be inferred as to other kinds

of the same class, with such modifications as distin-

guish different species of the same genus.

There is ample proof that heat is a mode of molec-

ular motion. Not that the heat produces the motion, or

the motion the heat, but that it is motion ; that the

molecules of matter being thrown into a certain kind

of motion, the result is the sensation known as heat.

According to the universally accepted theory of

light, it is another species of motion of the same

kind; and there are indications that light and elec-

tricity are identical. But, if not identical, we may

at least assume that they are closely allied to each

other.

We find also that the same causes, acting at the

same time, on the same bodies, and under the same


circumstances, produce both heat and electricity, in

numerous instances; in others, equally numerous, both

heat and light, and in others, heat, light, and elec-

tricity.

The simultaneous production of heat and electric-

ity is seen in the examples already given of bodies

electrified by friction, of which heat is also a neces-

sary result.

Another prominent instance is the action of the

electric generators known as dynamos ; in which the

evolution of heat is such that special provision for cool-

ing has to be made, to prevent injury. Here mechani-

cal action is the accent.

In the galvanic batten* we have a well-known in-

stance of the production of heat and electricity by

chemical action ; as a certain amount of heat, more or

less perceptible, is always a result.

Instances of the simultaneous production of heat

and light are numerous and well known, as the heat-

ing of an iron rod, which becomes luminous when the

temperature rises to a certain degree ; whether it be

heated by friction, as of a shaft and journal, or by the

chemical action of a furnace.

The dynamo and galvanic battery have been re-

ferred to as producing both heat and electricity.

When a current of this electricity, of sufficient in-

tensity, is passed through a conductor of high resist-

ance, as a fine platinum wire, or a carbon filament, theybecome luminous by incandescence; and when passedthrough two sticks of carbon slightly separated, we havelight of great intensity : and there is, in both instances,the evolution of intense heat. These are perhaps the

most striking examples which can be given of the sim-


ultaneous evolution of heat, light, and electricity from

the same causes.

In the thermo-electric battery we have an example

of the direct production of electricity by heat.

Polarized Light axd Electkiclty.—Experiments

with polarized light, made by Faraday and others, fur-

nish strong evidence of the close alliance, if not actual

identity, of light and electricity.

It is known that ]ight, from certain peculiarities of

reflection and transmission, undergoes a change, so

that when subsequently transmitted, its action is dif-

ferent from that of the original transmission, and this

change has been termed polarization.

Let a plate of tourmaline be so placed that a ray of

light falling on it shall be transmitted at right angles

to a particular direction within the crystal, known as

its optical axis. Then let another tourmaline plate be

so placed with reference to this one that their optical

axes are parallel, and that the ray shall pass through

both and form a bright spot on a screen beyond.

Now let either plate be turned, so that their optical

axes are no longer parallel to each other, but still at

right angles to the ray ; the bright spot on the screen

will fade as the angle increases, till at 90 degrees it will

disappear. Continuing the rotation, it will re-appear,

increasing in brightness, till, at 180 degrees, it is en-

tirely restored ; then fading again till extinguished

at the end of the third quadrant, and again increasing

in brightness till restored at the end of the fourth

quadrant, or original position.

This alternation of brightness and extinction de-

pends on the relative angular position of the plates.

Let either of them be turned in either direction, and


the same result follows ; but when both are turned in

the same direction, there is no change in the brightness,

and no extinction of the light. Substitute one for the

other, and the same results are obtained.

It is evident, then, that the light in passing through the

first plate has undergone a change which affects its trans-

mission through the second, in any position except whenthe optical axes of both are parallel; extinguishing it

entirehT when they are at right angles to each other.

It is also found that this effect, termed polarization,

occurs to light transmitted through or reflected from any

transparent medium, as glass, selenite, Iceland spar,

and various liquids. Polished metals also produce the

same effect on reflected light. And this reflection or

transmission takes place at a certain angle, known as

the polarizing angle, which varies in each substance

by a certain definite amount.

Now let the ray be transmitted through, or reflected

from, a small piece of glass of suitable size or shape,

placed at the proper polarizing angle, and let the plates

be turned till their optical axes are at right angles, so

as to produce extinction ; then let the glass be sub-

jected to a powerful electric strain and the extinguished

light will re-appear, continue during the electric action.

and disappear when it ceases; which shows that this

electric action has counteracted the effects of polar-

ization.

Similar experiments with various substances, too

numerous to detail here, show similar result-.

Without anticipating another branch of our subject,

it may 1)0 stated here, that electricity and magnetismare so closely allied that whatever affects one musthave some important relation to the ether.


Recent experiments have demonstrated that unusual

disturbances in the variation of the magnetic needle

are coincident with unusual disturbances in the sun, in

connection with the phenomena known as sun spots

;

and that the telegraph and telephone are, at such

times, seriously disturbed by what are known, techni-

cally, as " electric storms"; that is, unusual disturb-

ances in the earth's electricity shown in the phenomena

known as earth currents.

It has also been found that these solar disturbances

are periodic; and that these periods, for the last hundred

and fifty years, correspond almost exactly with the

periods of unusual variation of the magnetic needle.

Since the sun is our chief source of light and heat,

since they are, in fact, the result of the constant dis-

turbance of the elements of that body; and since, whenthis disturbance assumes an unusual phase, there is, co-

incident with it, an unusual disturbance in the earth's

electricity, it must be accepted as strong proof of a

common origin of heat, light, and electricity.

We have heat, light, and electricity derived from

friction, from chemical action, from magnetic action,

and from the sun. We have heat producing electric-

ity, and electricity producing heat and light ; and we

have electricity neutralizing the polarizing effect of

light. The evidence of identity then becomes cumu-

lative, while that of close alliance amounts to demon-

stration. Hence we may infer certain facts in regard

to the nature of electricity from what we know of

similar facts in regard to the nature of light and heat.

And we are also warranted in the conclusion, that a

well supported theory of light or heat requires but

little modification to adapt it to electricity.


This much we certainly know, that they all are forms

of energy and that they radiate from the centers where

they are generated. Hence the term radiant energy

is equally applicable to each.

The Wave Theory.—It has been assumed that a

subtle medium, termed ether, pervades all space ; that

it is so attenuated that it can insinuate itself between

the grosser molecules of material bodies; so that solids

of the finest and closest texture, as well as liquids and

gases, are pervaded by it; and that light, and probably

electricity, are due to waves or undulations of this

ether. The evidence of the existence of such a mediumis almost wholly negative, and, like all negative evi-

dence, unsatisfactory. The assumption presupposes the

necessity of its existence.

It has been stated that energy is a universal property

of matter, and the converse may be accepted, that

energy cannot exist without matter. And since light,

coming from the sun, must traverse the interplane-

tary spaces, there must be matter there ; else we shall

be compelled to admit that energy can exist without

matter, which is contrary to all our experience.

The earth is surrounded by an atmosphere, to the

limits of which we cannot penetrate. In 1822, Dr.

Wollaston made a careful mathematical calculation, as

the result of which lie claimed to have demonstratedthat the earth's atmosphere must have limits, beyondwhich it cannot exist. And this apparent demonstra-tion was accepted as authority, and remained unchal-lenged for half a century. Meantime the w;ive theoryoi light became prominent, and a medium being one of

its fundamental principles, the existence of the etherwas assumed, and is now generally accepted.


But the researches of modern science have swept

away many of the errors of the past, and it is not im-

possible that Dr. Wollaston's demonstration may share

the same fate. Many eminent scientists, who have

made experimental investigations on the subject, hold

that the expansibility of the earth's atmosphere is

unlimited ; among whom may be cited Grove, Gassiot,

Geissler, and Dr. Andrews. And W. M. Williams,

in his work, " The Fuel of the Sun," claims to have

discovered a serious error in Dr. Wollaston's calcula-

tions, which vitiates his conclusion.

The assumption of these writers is that an atmos-

phere, the same as that of our earth, pervades all

space ; that in the interplanetary spaces it becomes ex-

ceedingly attenuated ; and that each of the heavenly

bodies attracts and surrounds itself with a portion of

it; the extent and density of which is in proportion to

the mass of the body.

The high degree of vacuum which can now be attained

by improvements in the air-pump, seems to demonstrate,

that while electricity will pass more freely through rare-

fied air, on account of the reduced resistance, than

through air of ordinary density, it must still have a

medium in which to travel ; and that its passage through

an absolute vacuum, or space devoid of any known ma-

terial substance, is highly improbable. But as the best

attainable vacuum is still only an approximation to an

absolute vacuum, the full demonstration of this point

has not yet been reached.

The existence then of some elastic medium, by which

the two forms of radiant energy, known as light and

heat, can traverse the interplanetary spaces, is not

questioned. Nor does the theory of the unlimited ex-


pansion of our atmosphere conflict at all with the

theory of the universal existence of ether, since

the theory of ether is that it permeates all material

substances.

The wave theory assumes that radiant energy is

transmitted by the undulations of some medium ; that

an impulse originating at any center of energy, as the

sun, produces a wave which traverses this medium with

inconceivable velocity, till it reaches some distant

point, as the earth ; and that the constancy of such im-

pulses at every point on the sun gives rise to the phe-

nomena of solar light, heat, and electricity.

In like manner we may assume any other center of

energy, as a red-hot metal ball, radiating light and

heat; a stick of ebonite, excited by friction, radiaiing

electricity.

It is also assumed that the impulses radiate in straight

lines, while the undulations occur at right angles to

those lines.

To illustrate :— Drop a pebble on a smooth sheet of

water; the impulse creates waves which radiate out-

ward in widening circles. The pebble has depressed

the water at the point where it struck, forcing the ad-

jacent water outward, and causing it to rise above the

general level; then the downward impulse of this

wave, linking under the force of gravity, raises the

original center, and also produces, by ks outward im-

pulse, another wave beyond, as it descends by its inertia

below the general level.

As the water oscillates vertically above and below the

level, each successive? impulse produces a new wave,

while the same process goes on in the outward waves,

creating new waves beyond, in ever widening circles,


till the force of the original impulse has been exhausted,

and the water returns to its original level.

Now it will be perceived that there is no transfer of

the water from the center outward bej^ond the length of

the first wave. Part of the water forced outward by the

original impulse flows back again, while another part

flows outward, producing a new wave. The water is then

under the influence of two forces, one horizontal, the

other vertical, acting at right angles to each other; the

horizontal producing the wave length, that is, the

distance from crest to crest, or from hollow to hollow,

while the vertical produces the height, that is, the

vertical distance from hollow to crest, or from crest to

hollow.

In a similar way, it is supposed, occur the undula-

tions of the assumed elastic medium, with this excep-

tion, that the waves on the water occur in the same

horizontal plane, radiating outward in concentric cir-

cles, while those in the elastic medium occur in any

direction in which they are free to move ; radiating

outward in concentric spheres, if wholly unrestrained

,

or in sections of spheres or spheroids, if limited and

starting from impulses at various points on any surface,

either spherical, like that of the sun, or plane.

Having taken an illustration from a liquid, illus-

trations from solids will also be in point.

If a long rope, stretched lengthwise, with plenty

of slack, be held at one end, and jerked rapidly up and

down, it will be thrown into waves, which will run

along its entire length.

Here it is evident that while the impulses given at

one end travel in waves to the other, the rope, as a

whole, remains stationary; successive portions acting


as yielding levers to transmit the impulse along its

length.

If one end of a lever be depressed below a horizontal,

it receives a forward as well as downward movement,

in the arc of a circle, its opposite end receiving an up-

ward and backward movement. In this way each suc-

cessive portion of the rope oscillates horizontally as

well as vertically, modified by the difference between a

yielding and a rigid body.

Let a number of elastic balls be suspended in a

straight line in contact with each other. Draw back

the outer ball at one end of the line and let it swing

against the adjoining ball ; the impulse will be trans-

mitted along the line, and the outer ball, at the other

end, will swing off to nearly the same distance as that

through which the first ball swung, all the others re-

maining stationary.

Here the impulse is transferred from ball to ball byvirtue of their elasticity. When number one impinges

on number two the impact changes its shape slightly to

that of a spheroid; as it resumes its shape it imparts the

impulse to number two, by which it is imparted to num-ber three, and so on through the line. But action andreaction being equal and opposite, there is no perceptible

movement till the last ball is reached, which swings off,

since there is no ball to react against it. The impulse

travels, but the line remains stationary.

Here, also, it will be perceived that there is a radial

force acting at right angles to the horizontal force,

much the same as would result from a similar impactif each ball were hollow, and its surface composed of an

infinite number of semicircles, joined at the points of

impact.


Now the molecules of a metal rod may be com-

pared to an infinite number of these lines of balls; and

it may be assumed that a beat impulse, or an electric

impulse, given at one end, moves along these lines

in some way analogous to that in which the impulse

moves along the lines of balls.

We are not obliged to confine ourselves to any spe-

cific method of movement; but may suppose a wave

movement, similar to that which takes place in the

slack rope, or on the water, if it shall seem to accord

best with known facts and phenomena.

In the polarization of light, it is supposed that the

waves assume a certain phase, in conformity with the

special arrangement of the molecules of the crystal.

Hence if the crystals are cut from the same block, and

placed in the same position, the phase will be the same

for each, and the light will pass through. But if the

second is turned at right angles to the first, the phase

produced by passing through the first will not be in con-

formity with the arrangement of the molecules in the

second, and hence the light cannot pass through.

Suppose the arrangement of the molecules to be in

layers, or strata, like the sheets composing a ream of

note-paper, placed in a vertical position; the waves

of ether would assume a vertical phase, and, meeting

the second crystal, placed in the same position, would

pass through. But if the second were turned, so as to

bring its strata to a horizontal position, the vertical

waves would be broken, and could not pass through.

Instead of the ether we may suppose the molecules

themselves thrown into waves, whose phase would con-

form to the structure of the crystal, and the same result

would evidently follow.

THE NATURE OF ELECTRICITY, 37

There is no reference, in this supposed case, to any

visible stratification of a crystal, as the experimental

ray is usually admitted at right angles to such stratifi-

cation ; the reference is to an invisible arrangement of

the molecules.

The same course of reasoning is applicable to heat,

or to electricity, but the phase of the heat wave, or

the electric wave, may be different from that of the

wave of light, so that a substance opaque to light, as

copper, might allow the free passage of heat or electric-

ity ; or a substance transparent to light, as glass,

might obstruct their passage.

And the medium in which the electric energy travels

may be the ether which is supposed to pervade the

different kinds of matter; or the matter itself, in any

of its various forms, solid, liquid, or gaseous: as it

has been shown that undulations may take place in

each of them.

Conductivity for Heat and Electricity Com-pared.—It is very remarkable, and must be something

more than mere coincidence, that conductivity for heat

and electricity is nearly the same in the same sub-

stances. A good heat conductor is a good electric

conductor; a non-conductor of heat is a non-conductor

of electricity. So that if we know either the conduc-

tivity or resistance of any substance for heat, we

have, ] ractically, its conductivity or its resistance for

electricity. This will appear from the table follow-

ing, showing ihe results obtained by Wiedmann and

Franz.

Hence if heat and light are modes of motion, travers-

ing various substances by undulations, we arc warranted

in assuming the same with reference to electricity.

38 elements of static electricity.

Comparative Conductivity of different sub-

stances FOR HEAT AND ELECTRICITY, AS GIVEN BYWlEDMANN AND FRANZ :

—

Substance. Heat Conductivity. Electric Conductivity.

Silver 100 100Copper 74 73Gold 53 59Brass 24 22Tin 15 23Iron 12 13Lead 9 11Platinum .... 8 10German silver . . 6 6Bismuth .... 2 2

Other observers place the electric conductivity of

some of these substances much higher, making the con-

ductivity of copper nearly equal to that of silver.

If the electric wave has its own peculiar structure, it

is evident that a substance whose structure is adapted

to it, or whose molecules easily adapt themselves to it,

would be a conductor; while a substance whose struct-

ure is not so adapted, or whose molecules resist such

adaptation, would be a non-conductor.

An attempt to insert a No. 36 screw into a No. 30

screw hole will fail, because the threads of the screws

are not adapted to each other. But let the same screw

be inserted into some yielding substance, as soft wood,

and the substance adapts itself to the structure of the

screw ; or, as we say, it cuts its own thread ; while a

rigid substance like iron resists such adaptation.

Something analogous to this may constitute the

difference between conductors and non-conductors,

and may also be the cause of other electric phenomena

of equal importance.


The Spark and Sxap.—As already stated, every

substance offers a certain degree of resistance to the

passage of electricity, and a result of this resistance

is the generation of heat, often accompanied with light,

and this effect varies as the resistance.

Platinum is a metal of high resistance, while that of

copper is very low; and a fine platinum wire will be

brought to a white heat by an electric current which

would scarcely change the temperature of a copper wire

of the same size.

Air offers such high resistance that the passage of

electricity through it always produces both heat and

light, in the form of a bright spark. This occurs not

only when an electric charge passes through several

inches of it, but through the thinnest film ; the pres-

ence of heat, as well as light, being demonstrated by the

lighting of gas by a spark less than i inch in length.

A sudden condensation of the air, forced forward

and laterally by the charge, has been suggested as

the probable cause. If such condensation takes place,

heat would certainly be the result, as when air is com-

pressed by mechanical means. And perhaps it wouldbe accompanied by light, though this is not probable,

as combustion and incandescence are the only knownmeans of producing artificial light besides that nowunder consideration, either of which would imply the

presence of some other substance besides air. But the

hypothesis seems to assume the passage of some material

substance through the air to produce the condensation,

of which there is no evidence.

But if, instead of condensation, we suppose undula-tions to take place, giving greal intensity of motion.

as the electric impulse, darting forward with incon-


ceivable velocity, suddenly forces the resisting air into

the phases of the electric waves ; then the generation

of those other modes of motion, known as heat and

light, might easily be the result.

A sharp sound, varying from an insignificant snap to

a deafening report, always accompanies the spark. Onthe condensation theory this is accounted for by the

sudden displacement and reflux of the air. But since

sound, like heat and light, is another mode of motion,

occurring chiefly in the air. it is evident that the wavetheory will best account for it ; the electric impulse

giving rise to these different modes of motion.

The Dual Theory.—We have already seen, in ex-

periments with the pith ball electroscope, that the balls

may be attracted and repelled by electrified glass, seal-

ing-wax, and various other substances.

Let an electrified glass rod approach one of the balls;

the ball is attracted, and, after contact, repelled. Nowlet an electrified stick of sealing-wax be brought near,

and the electrified ball, which was repelled by the glass,

is attracted by the wax. Or let the ball be first elec-

trified and repelled by the wax, and it is attracted by

the glass.

Further experiments show that the same results can

be produced with other substances; glass representing

a certain class of substances, which show similar electri-

fication, and sealing-wax and resin another class, which

shows opposite electrification to that of glass.

Hence it has been assumed that there are two kinds of

electricity. One kind generated on the class of sub-

stances represented by glass, and the other on the class

represented by resin. The former was once designated

as vitreous, and the latter as resinous; but the term


positive is now used instead of vitreous, and negative

instead of resinous. Used in this way, these terms

have no reference to a difference either in quantity or

intensity ; they express only a supposed difference in

kind, not in degree.

This doctrine of the dual nature of electricity was

first proposed by Dufaye, and has since been strongly

maintained by many eminent scientists. Deschanel,

speaking of the phenomena under consideration, says :

" These phenomena clearly show that the electricity de-

veloped on the resin is not of the same kind as the elec-

tricity developed on the glass."

Now the only thing "clearly shown " is the difference

in the substances, not in the electricity. For we have

precisely the same electric phenomena of attraction

and repulsion with the glass as with the sealing-wax;

but a third substance, the electrified pith ball, is at-

tracted by one and repelled by the other; a result

which it would seem more reasonable to attribute to

the difference knoivn to exist between the substances,

than to a difference supposed to exist in the electricity.

For it has already been shown that different causes, as

conductivity or resistance, influence the intensity of

electrification on different substances. Other causesalso, as a difference of temperature or mass, of hard-ness or softness, density or porosity, doubtless contribute

to the same result.

But considering the quality of resistance alone, the

potential of any non-conductor, as glass, is liable to

vary greatly on different parts of its surface, whenelectrified by friction

; and to differ from the potentialof sealing-wax, similarly produced on different parts

of its surface.


The friction of the same rubber is also greater on

a substance like sealing-wax, whose surface is soon soft-

ened by the heat generated, than on a smooth, hard sub-

stance like glass, which is not affected in this way.

And, as already shown, difference of potential produces

attraction, while equality of potential produces repulsion,

between bodies. Hence the attraction by the sealing-

wax, of the pith ball electrified by the glass, is a neces-

sary result of difference of potential ; while its repul-

sion by the glass follows from equality of potential.

And the same will be true of the ball electrified and re-

pelled by the wax and attracted by the glass.

But if a difference exists in the kind of electricity

produced by the different classes of substances, Ave

should expect that difference always to manifest itself

whenever one of either class is employed as a generator.

But this is only true in a general way, to which the

exceptions are very numerous ; for it often happens that

glass and sealing-wax, or other substances belonging to

the different classes, when rubbed with the same rubber,

exhibit the same electric qualities. The same result

also will often follow where different kinds of rubbers

are employed, as silk on one substance and woolen on

the other.

Such results are inconsistent with the theory of two

electricities ; but are easily accounted for by a differ-

ence, or an equality of potential, which we know is

liable to exist.

Hence, preference must be given to the doctrine of one

electricity, originally proposed by Franklin ; simple and

plain, like truth itself, and in strict accord with all elec-

tric phenomena; whether pertaining to static electric-

ity, or to electricity under other forms.

CHAPTER IV.

Induction.

It is noticeable that in all our experiments thus far

the electrified body acts on the other bodies before there

is any actual contact. The knife-handle attracts the

spoon ; the sealing-wax, ebonite, or glass attracts the

balanced rod or the pith ball while separated from them.

And when either of these electrified bodies approaches

the gold-leaf electroscope, there is first a divergence of

the leaves before contact occurs.

It will also be noticed that this effect increases or

diminishes as the distance is increased or diminished.

And, further, that while the interposition of different

substances, as glass, paraffin, ebonite, air, wood, metal,

produce great variations in the effect, none of them

wholly prevent it.

There is evidently, then, an invisible influence ex-

tending to a certain distance from the electrified body

in every direction, and affecting everything within its

sphere, and this effect is called induction

When an electrified body is brought near the disc of

the electroscope without touching it. the leaves diverge,

and on its removal converge again, showing no perma-

nent effect. But if it is allowed to touch the disc, the

leaves are electrified, and remain divergent after its

removal.

But if, instead of touching the disc, it be held near


enough to produce divergence, as at J., Fig. 5, and, while

in that position, the disc be touched with the finger, as

at B, the leaves will converge, and remain so as long

as the electrified body is held near ; but on its removal

as at (7, they will diverge, and remain divergent, the

same as after contact of the electrified body with the

disc.

Fig. 5—Induction Illustrated.

Here, then, is electrification by induction, without

any transfer of electricity by contact. How can this be

accounted for ?

When the electrified body is brought near, whether

its charge be positive or negative, the effect of induction

is to produce a temporary change of the potential of

the electroscope, and the leaves diverge.

If the charge of the electrified body be positive, elec-

tricity is repelled from the disc to the leaves, and they

diverge, being positively electrified to the same poten-

tial, and hence mutually repellent, and also attracted

by the lower potential of surrounding bodies.

But if the electrified body be negatively charged,

INDUCTION. 45

electricity is attracted from the leaves to the disc, and

they diverge, being negatively electrified, and mutually

repellent, as before, and attracted by the higher poten-

tial of surrounding bodies.

Now, when the disc is touched with the linger, and

thus connected with the earth, if the charge is positive,

the potential of the electroscope is changed by the

escape of electricity to the earth under the influence of

the electrified body, and the leaves converge. But if

the charge is negative, the potential of the electroscope

is changed by the attraction of electricity from the earth,

and the leaves converge as before, equilibrium being re-

stored between the disc and leaves in each case.

The leaves remain convergent so long as the electrified

body is held near ; the electroscope being still under the

influence of the force by which the change of potential

was produced ; which is evidently just equal to the re-

pelled energy in the first instance, and to the attracted

energy in the second. But when the electrified body is

removed, this equilibrium is disturbed, and the leaves

diverge under the influence of mutual repulsion and

outward attraction, as already explained.

This experiment proves that a body connected with

the earth, and under the influence of induction, maydiffer in potential from the earth, and is not necessarily

at zero potential from its earth connection. For it is

evident that such difference of potential existed during

the connection of the electroscope with the earth, else

it could not have become manifest when the connection

was severed and the inductive influence removed. Forwhen the electrified body is removed before such con-

nection, the leaves converge, but when removed after it

has been made and severed, they remain divergent;


showing that the difference of potential was created and

existed during the earth connection.

This point has an important bearing on phenomena to

be considered hereafter, in regard to which prominent

writers have been betrayed into serious mistakes from

having overlooked it.

Influence of Distance.—It is important to notice,

that when the electroscope has been charged b}~ induc-

tion in this manner, and the electrified body is again

brought near, the leaves continue to converge as the

body approaches, and come together when it is in the

same position as when the disc was touched with the

finger. A nearer approach produces divergence, which

increases as the body is brought still nearer.

Let it now be gradually withdrawn, and the leaves

gradually converge and come together, when the body

reaches the same point, as before. Further withdrawal

produces divergence, which continues to increase, and

reaches its limit when the body is wholly removed.

If the electroscope be placed at various points, equally

distant from the electrified body, the effect of induction

will be the same, so long as the same distance from the

earth and surrounding objects is maintained. Henceit is evident that electric energy, like other forms of

radiant energy, as light and heat, radiates equally

in all directions when not interfered with by other

influences.

Suppose the electrified body to be a small globe,

entirely removed from the earth, and surrounded with

a perfectly homogeneous medium, it would be the

center of a sphere of inductive influence. And suppose

the lines of force radiating from it to be cut by the

surfaces of two imaginary concentric spheres of differ-

INDUCTION. 47

ent sizes, one placed outside of the other, and having

the electrified globe for their common center.

Since the surfaces of spheres are to each other

as the squares of their radii, and since the radii meas-

ure the distances from the center, the surfaces are to

each other as the squares of their distances from the

center.

But as each surface embraces all the lines of force,

the intensity of force on equal surface areas of the two

spheres would be in the inverse ratio of their entire sur-

faces ; and hence would vary inversely as the squares

of their distances from the center.

Hence, electric induction varies inversely as the square

of the distance.

Practically the conditions of the supposed case are

never exactly fulfilled ; but that does not affect the

correctness of the principle, which is the same in elec-

tricity as in light and radiant heat.

Cylinder Electrified by Induction.—The effect

of induction may be further illustrated by an insulated

cylinder of conducting material, placed between two

spheres of similar material, one insulated, and the other

connected with the earth by a chain, as shown in Fig.

6; the cylinder having mounted on it three pith-ball

elec es, connected with it by conductors.

It* the insulated sphere, .1, be positively electrified,

electricity will be repelled by induction from the end of

the cylinder next A to the end next //. And since Bis connected with the earth, the electricity accumulated

on the end of C\ next to it, will repel to the earth from

B an amount equal to the positive charge? on A.

Hence the pith ball next .1, being negative and A

positive, is attracted by A%while the one next />, being


positive and B negative, is attracted by B ; but the

central ball, being neutral, remains unmoved.

If the sphere, A, be negatively electrified, these condi-

tions will all be reversed. Electricity will be attracted

to the end of C next A, and a positive charge, equal

to the negative on A, attracted from the earth to B.

Hence the balls will assume the same positions as

before.

Fig. G—Cylinder Electrified by Induction.

Similar inductive effects can be produced on the cyl-

inder by the sphere A alone, but less marked than whentwo spheres are used; and, for such an experiment, tin-

foil electroscopes are better than those made with pith

balls, being more sensitive.

Theory of Induction.—It Is not known how in-

ductive force is transmitted. The hypothesis has been

advanced that it is by a certain strain of the medium;

as when a weight is lifted by a rope or pushed by a pole,

the energy is transmitted in one case by the tension of

successive portions of the rope, and in the other by a

compression of successive portions of the pole. In

either case the energy or stress produces a strain, which

INDUCTION. 49

runs through the substance of the medium till it reaches

the object, and the continued stress produces continued

strain. Something analogous to this, it is assumed,

takes place in the transmission of electric energy by

induction.

This hypothesis has the sanction of eminent author-

ity, and may assist us in arriving at a solution of the

problem. Gordon says: "If electric induction were a

6 direct action at a distance,' we should expect that it

'would be transmitted equally through all insulators.

One of the strongest arguments for supposing it to be

a strain of the particles of the insulator is found in the

fact that different insulators transmit it with very differ-

ent strengths."

"Induction, so far from being a 'direct action at a

distance,' is most certainly transmitted by the particles

of the dielectrics, and is affected by almost every molec-

ular change which may occur in them."

And he defines strain, as here used, to mean " an

alteration of size or shape," including "all alterations

of volume," "all twistings and bendings, and all vibra-

tory motions other than those of a rigid body as a

whole."

The wave theory agrees with the views here expressed

;

for we have only to conceive that this "strain" consists

in a "vibratory motion," that is, in undulations of the

medium.

It is also in accordance with the analogy of similar

transmission of other forms of radiant energy. And, if

all energy has a common origin, it is reasonable to sup-

pose that the transmission of its different forms wouldpresent striking analogies.

Influence of Dielectric.—In order to observe


induction there must be two or more bodies at different

potentials placed in each other's vicinity, and these must

be separated by an insulator; for, if separated by a con-

ductor, equilibrium would at once be restored, and in-

duction could not take place.

Insulators through which induction takes place are

called dielectrics, from the Greek &«, through. Air was

the dielectric between the electroscope and electrified

body, and between the spheres and cylinder, in the ex-

periments already given.

Now, since conductors permit electricity to pass

through them easily, while insulators resist its passage,

there must be some peculiarity in the nature or arrange-

ment of the molecules which makes two bodies of the

same class similar in this respect, while two of opposite

classes are dissimilar.

Hence we can easily conceive that when two insulated

conductors, at different potentials, are brought into con-

tact, the undulations of their molecules would assume

the same phase, and equilibrium take place; but that

when those undulations are transmitted through a die-

lectric, they undergo such a change that, the phases of

the undulations not being the same, there is a repulsion

instead of an intermingling, which results in creating

opposite potentials in adjacent parts, on either side of

the dielectric, the negative of one being equal to the

positive of the other.

And since in the transmission, part of the energy is

consumed in overcoming the resistance, difference of

potential, on opposite sides, must result from this cause

also.

If either conductor be removed, still remaining insu-

lated, the equilibrium of each will be restored, and its

IXDUCTION. 51

potential be found the same as before it was brought

within the sphere of inductive influence, showing that

no permanent effect has resulted.

Hence it will be seen that the effect of induction is

opposite to that of contact; the latter producing perma-

nent equilibrium between conductors, while the former

produces temporary disturbance of equilibrium.

Specific Ixductive Capacity.—It has already been

stated that electric induction takes place through all

substances, but in different degrees ; and, since it

is found that each has an inductive power peculiar

to itself, this property is called its specific inductive

capacity.

The importance of this subject will be understood

when it is considered that it affects enterprises in-

volving large capital, public convenience, and public

safety : as in the transmission of electric energy byinsulated conductors, as telegraph and telephone wires,

ocean cables, and electric light wires; including the

important question of underground transmission in

cities.

Hence, for the last forty years, it has engaged the atten-

tion of such men as Faradaj^, Boltzmann, and manyothers, including the earlier researches of Cavendish,

who to have been the first to investigate it, but

whose experiments on this subject have only recently

been published.

The genera] method of investigation is as follows:

—

The inductive capacity of dry air at the barometric

pressure of 7<>0 millimeters (29.92 inches) and at the

temperature of 0° C. (32 Fahrenheit) is made the

standard unit by which the inductive capacities of all

other substances are estimated.


To illustrate :—Suppose we have two insulated metal

plates, A and B (Fig. 1), separated by an air space C;let A be electrified and B connected with tlie disc of

an electroscope. First note the amount of divergence

of the leaves ; then let a plate of glass, cake of paraffin,

or any other insulator which will exactly fill the space

Fig. 7—Specific Inductive Capacity Illustrated.

(7, be introduced between the plates, and note the diver-

gence of the leaves now, as compared with the former

divergence.

As this insulator has displaced the air, it is evident

that its inductive capacity, as compared with air, is

shown by the difference in the divergence of the leaves.

INDUCTION. 53

If that divergence has increased, then the power of this

insulator to transmit electric influence—that is, its spe-

cific inductive capacity—is greater than that of air;

otherwise, it is equal to, or less than that of air.

From this we see that specific inductive capacity varies

inversely as insulation. Hence this property is almost

infinite in the best conductors ; while in the best insu-

lators it is the reverse.

By methods similar to the above, with the aid of

improved instruments, to be described hereafter, the

specific inductive capacities of a number of substances,

including the principal insulators, have been carefully

estimated by Boltzmann, Gordon, and others: and from

the results obtained by them the table on the next page

has been prepared, in which the general averages are

given.

The results obtained by different observers differ so

widely that they can onl}r be regarded as approximate,

and will undoubtedly require future correction, whenimproved methods shall give greater accuracy.

The table shows the electric resistance of glass to be

much less than that of ebonite ; the inverse ratio being

5.87 to 2 89 : and this is doubtless true of glass, in the

average. But, if the best insulating glass were com-

pared witli the best insulating ebonite, the ratio might

require to be reversed. Ebonite, Avhen subjected to a

powerful electric strain, seems to yield gradually, andallow the electricity to creep through it ; and, by con-

tinued strain, its electric resistance soon becomespermanently impaired: while the best insulating glass

rigidly resists, and suffers fracture before yielding.

But, according to Gordon, the electric resistance of

glass also becomes somewhat impaired by long use ; or,


which is the same thing, its specific inductive capacity

is increased. All of which goes to prove that electric

transmission depends on molecular structure.

Specific Ixductive Capacities of VariousSubstances.

Standard.

Air at 0° C. temperature and 760 mm. pressure, 1.0

Solids.

Paraffin, 2.09

Caoutchouc, 2.23

Gutta-percha, 2.46

Shellac, 2.85

Ebonite, 2 89

Sulphur, 2.95

Resin, 8.6

Glass, average of various kinds, 5.87

Liquids.

Bisulphide of carbon, 1.81

Petroleum, 2.05

Oil of turpentine, 2.19

Gfases.

Hydrogen, H, at 0° c. and 760 mm ., .99941

Carbonic oxide, CO, U u 1.00001

Marsh gas, CH 4,

u t6 1.00035

Carbonic dioxide, co 2,u CC 1 00036

Nitrous oxide, NO, u OC 1.00039

Olefiant gas, C2H4 ,

U " 1.00072

CHAPTER V.

Electric Distribution and Condensation.

Equipotential.—A charge of electricity given to

any part of a good conducting surface is instantly dis-

tributed equally over every part, and such a surface is

called equipotential. For the momentary increase of

electric energy at any point creates electric movementfrom higher to lower potential, which instantly results

in the establishment of equilibrium at every point.

Separate points on such a surface are called equipo-

tential points, and a line of such points an equipotential

line.

Lines of Force.—The direction along which elec-

tricity tends to move, from a point of higher to one of

lower potential, is called a line of force. Such lines

are perpendicular to the equipotential surfaces at the

points ; for, as the tendency is to move from one point

to the other, it would be from one such surface to the

other; and if the line differed from a perpendicular, it

would imply, by the resolution of forces, that there

could be two lines of force at right angles to each

other, one of which would lie in an equipotential sur-

face ; implying two points at different potentials in such

surface, which would be an impossibility.

Surface Condensation.—Since the surface of a

solid sphere of any good conducting material is evi-

dently equipotential, we may regard its interior as


composed of an infinite number of such surfaces, or

spherical shells, having a common center ; and their

radii as equipotential lines cut by such surfaces. Fromwhich it is evident that no difference of potential could

exist in the interior of such a sphere.

If it were insulated, a positive charge communicated

to it would evidently be distributed equally through

every part, if there were no influence tending to pro-

duce a different effect. But, since the sphere would

be at a higher potential than its surroundings, induc-

tion would create lines of force in the direction of the

radii, which must result in the condensation of the en-

tire charge on the surface.

Also, since every portion of the sphere is at the

same potential, and since electrified bodies at the same

potential repel each other, it is evident that the mole-

cules would be self-repellent. But since they are rigid,

the electricity of each molecule would repel that of

every other, and move in the direction of least resist-

ance. Let a row of molecules composing a diameter

be selected, the direction of least resistance would be

from the center each waj^. For, if surface condensa-

tion takes place (and experiment shows that it does),

as the electricity of the molecules near each end of the

diameter became condensed at the extreme points, its

reaction being thus neutralized, more would be repelled

from the center, and this would continue till all the

electricity of the diameter was condensed at the ends.

But since the ends are points on the surface, and the

surface is made up of an infinite number of such points,

it is evident that the entire charge would be condensed

on the surface.

Hence surface condensation takes place under the

ELECTRIC DISTRIBUTION AND CONDENSATION 57

influence of attraction from without and repulsion from

within, in the direction of the radii.

If the charge be negative, the potential of surround-

ing bodies being higher than that of the sphere, elec-

tricity is, in like manner, repelled from the surface

toward the center ; and the negative charge takes place

on the surface, as the positive charge did in the first

instance. Hence the condensation is now in the inte-

rior, leaving the surface negative.

Hence surface charge, if positive, takes place under

the influence of attraction from without and repulsion

from within ; but, if negative, under the influence of

repulsion from without.

In either case the air is the dielectric between the

electrified sphere and surrounding bodies : and whenthe charge on the sphere is positive, a negative charge

of corresponding amount is induced on adjacent parts

of surrounding bodies ; electricity being repelled from

them by the higher potential of the sphere. But whenthe charge on the sphere is negative, the charge on

adjacent parts of surrounding bodies is positive ; elec-

tricity being attracted to them by the lower potential

of the sphere.

Now since surrounding bodies, as a whole, are at

zero, and this positive charge, in their adjacent parts,

results from the negative attraction of the sphere, it

is evident that the interior potential of the sphere, as a

whole, cannot rise above zero; the negative potential

of its surface being exactly equal to the positive of

adjacent parts of surrounding bodies, just as their

negative potential was equal to the spline's positive

surface potential in the first instance. Now, since a

solid of any conceivable shape could be cut from such a


sphere without altering the electrical conditions named,

it is evident that, A charge of electricity communicated

to any solid conductor will be condensed on its surface.

Surface Transmission.—It is also evident, that

although a static charge will be thus condensed on the

surface, electric transmission is not confined to the sur-

face : since surface condensation is due to induction

and repulsion, which implies the possibility of trans-

mission through the substance to reach the surface.

Hence, although induction operates during transmis-

sion, it cannot prevent transmission through the sub-

stance : so that it must not be inferred that the con-

ducting power is in proportion to the surface, but to the

mass of the conductor.

Hence a charge of electricity which could be easily

transmitted by a solid rod might be sufficient to melt

a thin tube of the same diameter.

Hollow Conductors.—The same reasoning which

applies to an electric charge on a solid sphere will also

apply to one on a hollow sphere. For if any number of

the spherical shells composing the interior be removed,

it does not alter the equipotential of the remaining

ones, nor of their radii ; neither can it change the induc-

tion of the outside surroundings.

And as the form may be altered without changing

these electric conditions, the same reasoning will apj3ly

to any hollow conductor.

Hence, A static electric charge, communicated to a hol-

loiv conductor, will be condensed on its external surface-

Proof Plane—But all our conclusions should be

the result of experiment ; to aid us in which we nowrequire the little instrument called the proof plane,

represented in Fig. 8 ; which consists of a small brass

ELECTRIC DISTRIBUTION AND CONDENSATION. 59

disc, two inches in diameter, to which is attached a

light ebonite handle, 12 inches long. A light, flat

spring, which lies close to the disc, its lower end free,

and its upper end attached to the handle, will be found

convenient for attaching tin-foil in some experiments.

aFig. 8—Proof Plane.

The proof plane is used for examining the electric

condition of bodies, and for transferring a small charge

of definite amount. Care should be used to prevent

the handle from becoming charged, which may happenfrom friction against the clothing or otherwise.

Experiments with Hollow Conductors.—Let a

charge of electricity be given to the insulated sphere A,

Fig. 9, which has an

opening in the top. In-

troduce the proof plane

through this opening,

taking care to prevent

contact with the edges;

and touch the inside sur-

face and then the disc

of the electroscope, with

it. As the leaves showno divergence, it proves

that the inside is not

electrified.Fi ^' ()~ II()1,ow Conductor.

Now touch the outside, and then the disc, and the

leaves diverge; proving that the charge is on the out-

side surface.

Apply the same tests to the insulated cylinder B, and


the same results will follow. And this cylinder maybe composed either of sheet metal or wire gauze without

affecting the results.

Cylinders of the latter kind are often used to protect

electroscopes from the induction of electrified bodies

in their vicinity.

Repeat these experiments, communicating the charge

to the inside surfaces of the globe and cylinder, and the

results will be the same ; showing that no charge can

remain on the inside.

Fig. 10—Faraday's Bag.

Bag Experiment.—To test this more thoroughly,

Faraday constructed a cone-shaped linen bag, shown in

Fig. 10 ; attached to its mouth a ring insulated on a

stand, and to its apex two silk cords, by which either

surface could be turned outward.

An electric charge was communicated to it, and, on

testing with the proof plane and electroscope, was found

to be entirely on the outer surface. The surfaces were

now reversed, and the charge was found to have been

reversed also, going to the outside, as before.

Pail Experiment.—The following experiment by

Faraday shows the effect of induction on a hollow con-

ductor :

Let a tin pail A, Fig. 11, or any similar hollow con-

ELECTRIC DISTRIBUTION- AND CONDENSATION. 61

ductor, be insulated and connected by a wire with an

electroscope _Er

, and let an electrified metal ball B be

lowered into it by a silk cord. The leaves will diverge

as the ball enters, and the divergence increase till the

ball has passed some distance below the edges : after

which the divergence is not increased by its further

descent.

Fig. 11—Pail Experiment.

If it be lifted out without having touched the pail,

the leaves will converge, and the ball show no loss of

charge : but, if allowed to touch while below the edge,


the leaves will remain divergent after its removal, but

show no increase of divergence by the contact; and the

ball, after removal, will be found entirely discharged.

This experiment proves :

—

1. That the induction of the electrified ball has re-

pelled electricity from the inner to the outer surface of

the pail if the charge was positive, or attracted elec-

tricity from the outer to the inner surface if the charge

was negative ; in either case producing a divergence of

the leaves.

2. It proves that induction increases as the ball de-

scends, shown by the increasing divergence of the leaves,

till all the lines of force, which can be included within

the pail, are cut by its surface, after which there is no

further increase of divergence.

3. It proves that there is no permanent effect if

there is no contact ; since the leaves converge when the

ball is removed.

4. It proves that the induced charge on the pail is

exactly equal to the charge on the ball, since no increase

of divergence occurs from contact, although the entire

charge has been communicated to the pail, as shown by

the ball having lost its charge. But this can be strictly

true only when all the lines of force are cut by the pail

;

but since some of the nearly vertical lines must escape,

no matter how deep the ball descends, there must be a

slight increase of divergence by contact, though it maynot be perceptible.

If a charge be given to the pail and the ball be low-

ered into it by a wire held in the hand, the divergence

of the leaves, caused by the charge on the pail, will be

perceptibly reduced as the ball descends.

This proves that the inner surface of a hollow con-


ductor can be charged by induction. The charge on

the pail, if positive, repels electricity from the ball,

through the wire and hand, to the earth ; or, if nega-

tive, attracts electricity from the earth; and in either

case, a certain degree of equilibrium follows, causing a

corresponding convergence of the leaves.

Entire convergence cannot be produced, since only

a small portion of the lines of force from the pail are

cut by the ball ; while, in the former experiment, nearly

all those from the ball were cut by the pail. For this

reason a large ball is best for the second experiment

and a small one for the first.

If the ball, in the second experiment, is lowered by a

silk cord instead of a wire, there is no perceptible effect

on the leaves, since induction cannot increase nor dimin-

ish the electricity of the ball when there is no earth

connection.

Combination of Patls.—The following experiment

was made by Faraday with a combination of hollow

conductors :

—

Let four pails of different sizes be placed on an insu-

lated support, and arranged one within the other as

shown in Fig. 12: and let them be insulated from each

other at bottom by cakes of paraffin, or any other goodinsulator, placed between them. Let silk cords be

attached to tin- three inner ones, and the outer one be

connected with an electroscope.

On lowering the charged ball into the innermost one,

the leaves diverge as in the first experiment; contact

between the ball and pail producing no increase of

divergence, and the ball is then found to be discharged,as before: which proves that the interposition of the

insulated pails, 2 and 3, has not affected the induction.


Now let pail No. 4 be lifted out by the silk cord, and

the leaves will converge, and diverge again when it is

replaced, showing that the charge on the ball was trans-

ferred to it.

Fig. 12—Combination of Pails.

Let a connection be now made by pieces of copper

wire, let down by silk threads, between each of the

pails successively, beginning with 4 and 3, till all four

are in electric connection, and let the effect on the

leaves be observed as each connection is made. The

results will be found the same as in the first experiment,


when but one pail was used: which proves that the

interposition of interior surfaces has no effect on induc-

tion ; nor can it prevent the entire charge from going

to the outside surface when the four pails are in electric

connection; for if the three inner pails be now removed,

they will be found to have lost their charge; but there

will be no change in the divergence of the leaves.

This experiment is an actual demonstration of what

has already been stated, that the interior of a solid con-

ductor, or the shell of a hollow conductor, may be

regarded as composed of an infinite number of equipo-

tential shells or surfaces, from which a charge of elec-

tricity must always pass to the outside surface.

Faraday's Hollow Cube.—A most remarkable ex-

periment in this connection was made by Faraday with

a hollow cube of wood, measuring twelve feet each way,

covered with tin-foil, insulated and charged by a power-

ful electric machine.

He says : " I went into this cube and lived in it,

using lighted candles, electrometers, and all other tests

of electrical states. I could not find the least influence

upon them, or indication of anything particular given

by them, though all the time the outside of the cube

was powerfully charged, and la,rge sparks and brushes

were darting off from every part of its outer surface."

This experiment verifies the statement made on page

12 in regard to zero potential; showing that howeverstrong the electrification, no indications of electric action

are perceptible within a space where there is perfect

equilibrium. So that even if tlie whole earth were as

powerfully charged, in proportion to its size, as Fara-

day's cube, we, who live on it, could perceive no electric

action, if the charge were as uniform as on the cube.


But if it be objected that the case is not parallel, see-

ing that we live on the surface, it must be rememberedthat we have an atmosphere above us which is a part of

the earth's matter; so that, although we live on the

solid surface, we do not live on the outer surface : and

the surface on which we live is practically equipoten-

tial over limited areas.

Faraday, evidently, might have generated electricity

with insulated instruments, inside the cube, and con-

densed it on insulated conductors, without either dis-

turbing the electric conditions by which he was sur-

rounded, or being prevented by them : just as we do

without disturbing the earth's electricity, or being pre-

vented by it. But any connection by a conductor,

between his instruments and the cube, would have

caused the charge to disappear; just as a similar con-

nection with the earth produces the same result.

Thickness of Electrified Sukface.—The idea

of surface condensation implies that an electrified sur-

face must be something more than a mere superficies.

It must have a certain degree of thickness, the elec-

tricity penetrating the conductor and surrounding air

to a certain depth, in proportion to the resistance of the

air, and the attraction or repulsion of the charge on

the conductor. Hence the amount of static charge

which may be condensed on a conductor, per unit of

surface, depends on the resistance of the air.

Convection.—It has already been shown that dry

air is one of the best insulators ; but, since it is a fluid,

its resistance cannot be so great as that of a solid of the

same insulating power; for the air molecules, in contact

with an electrified surface, becoming charged, fly off

under the influence of repulsion and induction, while


those farther out rush in to take their place ; creating

air currents around the conductor, by which its elec-

tricity is gradually dissipated. The removal of electric-

ity by the air in this way is called convection.

Variation of Charge.— Since the insulating power

of the air varies greatly with its humidity and tempera-

ture, and since its electric potential is also variable, the

charge which may be condensed on a conductor will

vary in like proportion ; dry, cold air being much more

favorable to the condensation of a hi^li charge than

damp, warm air ; and air at a high electric potential

than air at a low potential.

• Analogous to this is the influence of atmospheric

pressure on steam ; the temperature varying with the

pressure under which it is generated. Here pressure

constitutes resistance, while in the case under consider-

ation the resistance is due to the causes mentioned.

Equal electric condensation on every part of the sur-

face is never practically true ; as the induction of sur-

roundings varies, and form, as will be shown hereafter,

lias an important influence. It could only be true of

an insulated sphere, surrounded by a homogeneousmedium, and removed from all other influences.

Influence of Form.—It lias already been stated

that form exercises an important influence on the

amount of static charge which may be condensed on a

conductor; and that a charge on an insulated sphere

is equally distributed over its surface, when the sur-

rounding induction is equal: also that the air, by its

insulation, retains this charge on the surface, andby its convection gradually removes it. It is evidentalso that these forces act at equal distances from thecenter.

68 ELEMEXTS OF STATIC ELECTRICITY.

Fig. 13—Spheres in Contact.

Electrified Spheres.—Let two insulated metal

spheres, of equal size and similarly charged, be placed

in contact, as represented in Fig. 13. It is evident that

either of them, separately, would fulfill the conditions

just named; but

when placed in con-

tact, they must be re-

garded as one mass,

having its center at

the point of contact

;

the electric distribu-

tion being the same

on each.

Hence the forces

of induction and re-

pulsion which before acted to remove electricity from

the center of the single sphere to the parts most remote

from it— that is, to the surface— now act in the same

manner, to remove electricity from this new center to

those parts of the mass most remote, that is, to the

points A and B, and the surfaces surrounding them.

There must also be a certain amount of electricity

distributed over the entire surface of each sphere ;and

there must be repulsion between the surfaces adjacent

to the point of contact : so that the charge will be zero

at this point, and increase each way toward A and B.

This may be demonstrated by touching the points

A, B, and with the proof plane, and. after each con-

tact, bringing it near the disc of the electroscope ;

taking care to discharge it with the finger before mak-

ing the next test.

It will be found that the central point shows scarcely

a trace of electricity, while the points A and B are


strongly electrified. The same test, applied to inter-

mediate points, shows the charge on them to be in pro-

portion to their distance from the central point.

Electeified Cylinder.—Instead of the two spheres,

we may substitute an insulated metal cylinder, with

hemispherical ends, provided with pith - ball electro-

scopes at the ends and center, as represented in Fig. 14.

A light charge of electricity

on the cylinder will cause the

balls at the ends to diverge in

opposite directions, while the

central ball will remain un-

moved, or but slightly affected ;

showing that the principal part

of the charge is condensed on

the ends, and that induction

and repulsion are operating to

remove electricity to the points

farthest from the center, as shown by the position of

the balls at the ends.

If a sphere be made to oscillate near one of the balls,

at right angles to the length of the cylinder, the effect

of induction will be shown by the ball following the

movement of the sphere.

INFLUENCE OF Points.—If a cylinder having cone-

shaped ends be substituted for the one with hemispher-

ical ends, dissipatioD of the charge, instead of condensa-

tion, will occur. For, on the hemispherical ends, the

charge is retained by the resistance of the air on the

surface; but the cone-shaped ends terminate in [joints

which have no surface, hence there can be no resistance.

But if resistance ls removed, even from a single point,

it is evident that the entire charge must pass off through

Fig. 14—Electrified Cylinder.

70 ELEMENTS OF STATIC ELECTRICITY,

that point ; since the removal of electricity from any

point on a surface creates a difference of potential be-

tween that and. surrounding points, producing an elec-

tric movement in the direction of the point of no

resistance, which must extend to every part of the sur-

face, and continue till equilibrium is restored.

Instead of the cylinder with cone-shaped ends, wemay use one with needles attached to the ends, as repre-

sented in Fig. 15. A wooden cylinder covered with

tin-foil can easily be changed in this way.

It will be impossible to

charge such a cylinder, even if

only a single needle be at-

tached to any part of the sur-

face. A projecting angle on

any part of a conductor will

tend to produce the same re-

sult.

Effects somewhat analogous

to these may be obtained by

dipping into water a spherical

body, and also a sharp-pointed

spike having the same amount of surface. On lifting

out the spherical body, water will adhere to it, and col-

lect in a large drop at the lowest part ; being held there

by adhesion and atmospheric pressure. But if the

spike be lifted out, point downwards, the water will

drop off when it reaches the lowest point, there being

no surface there on which it can be retained by those

forces.

Electrified Spheuoid.—If a metal sphere be flat-

tened at the poles till it assumes the form of an oblate

spheroid, as shown at A, Fig. 16, the face of a cross-

-Cylinder with PointsAttached.


SC J

section through the poles, as shown at B, will have the

same form as a cylinder with hemispherical ends. Andsince it has been shown that a charge of electricity on

such a cylinder is condensed

on the ends, it is evident that

a charge on such a spheroid

will, in like manner, be con-

densed on its outer edge.

Electrified Disc. — If a

flat metal disc, with a thin

edge, be electrified, the charge

will go to the outer edge, as in

the last case. But resistance,

being in proportion to surface,

is very small on such an edge,Fig

'16"Electrified Spheroid,

and the charge is rapidly dissipated. Hence such a

disc, when constructed for the purpose of condensing

electricity on it, should be pro-

vided with a round rim, which

may be called a resistance rim.

If it be insulated, and there

be placed on its opposite sides,

near the edge, two little metal

stands with pointed stems, on

which are balanced light metal

[jointers, having arms of un-

equal length, as shown in Fig.

IT. a charge of electricity given to it will cause the

pointers to arrange themselves in the direction of the

radii, showing that the electric force is from the center

outward.

17—Electrilied Disc.

CHAPTER VI.

Accumulators.

The Charged Pane.—The electric charge which

may be condensed on the surface of an insulated con-

ductor is comparatively small, when such a conductor

ris remote from inductive influence.

But when another conductor, having a connection

with the earth, is placed in its immediate vicinity,

the charge may be greatly increased.

To prove this, let a sheet of good insulating glass,

varnished with shellac, be

coated on opposite sides with

tin-foil, to within about two

inches of its edge, and placed

on an insulating support, as

shown in Fig. 18. A small

charge can be given to the

tin-foil, on the upper surface,

which will be indicated by sparks passing between it

and the body from which the charge is given. But the

limit is soon reached, and no more sparks will pass.

Now let the lower surface be connected with the earth

by a strip of tin-foil, and sparks will again pass freely

between the charging body and the upper surface, till

a charge greatly in excess of the former is given.

If the tin-foil strip be suspended with its lower end

near a conductor, as shown, sparks will pass between

The Charged Paue.

ACCUMULATORS. 73

it and the conductor, simultaneously with the sparks

on the upper surface ; indicating that each surface re-

ceives the same amount of charge.

But the potential on opposite surfaces will be oppo-

site. If the upper surface acquires positive potential,

by an increase of electricity, the same amount will be

repelled from the lower surface, making it negative.

But if the upper becomes negative by a decrease,

electricity, to the same amount, will be attracted to the

lower surface, making it positive.

To prove that these charges are equal, let the tin-

foil strip be removed

after the plate has been

charged ; and a wire,

held by a piece of

india - rubber tube, to

insulate it, be bent so

that its ends come into

contact with the oppo-

site surfaces, as shownin Fig. 19: a flash and

report will follow, and both surfaces, after the wire

has remained in contact for a few moments, will be

found completely discharged.

Now, since the removal of the strip produced com-

plete insulation, perfect equilibrium could occur only

by the positive of one surface being exactly equal to

the negative of the other.

Since induction varies inversely as the square of

the distance (page 47), it Is evident that, if this factor

alone is considered, the amount of charge which can be

given will be in the inverse ratio of the thickness of

the glass, and hence greater on thin than on thick

Fig. 19—The Pane Discharged.


glass. But since the resistance of glass is in the

direct ratio of its thickness, when the specific induct-

ive capacity is the same, this factor also must be

considered.

Hence, in the construction of instruments involving

these principles, if great sensitiveness and a low poten-

tial is desired, the glass, or other dielectric, should be

thin : but if the highest attainable potential is desired,

there should be sufficient thickness to resist fracture

or puncture.

The uncoated margin must also be wide enough to

make the resistance there equal to that of the thickness;

a small fraction of an inch in thickness having a re-

sistance equal to that of several inches of surface.

No definite rules can be given, as the resistance of va-

rious kinds of glass, and other dielectrics, varies greatly,

as well as the cases in which they may be required.

As the positive and negative on opposite surfaces are

equal, it is impossible for a change of potential to occur

on either surface without a corresponding change on

the opposite surface. Hence a conductor brought into

contact with either surface alone will not change its

potential, unless directly or indirectly connected with

the opposite surface. Hence the charge on each surface

is said to be bound by the opposite charge.

The convection and conduction of the air, so far

as it can act equally on both surfaces, will in time re-

store equilibrium. It may also be restored by the oscil-

lation of a solid bodjT, as a pith ball, suspended between

conductors connected with both surfaces ; or, by direct

connection through a conductor, as already explained.

Instruments constructed for accumulating electricity

in this way are called accumulators, or condensers.

AGCUMULA TOES. 75

The Leydex Jar.—The first discovery of an accu-

mulator was made by Kleist, a clergyman of Cammin,

in Pommerania. who stated in a letter to Dr. Lieber-

kiilm, of Berlin. Nov. 4. 1745, that by pouring a little

mercury. " spirits," or water, into a phial and con-

necting it with a nail through the cork, he could

electrify it through the nail, ignite "spirits of wine"'

with it, and receive a shock bv touching the nail with

his finder.

The same discovery was made in the following year

in Leyden, by Cuneus, a pupil of Musschenbroek, whoelectrified some water in a flask, which he held in his

hand, by bringing it into contact with a chain from the

conductor of an electric machine. On attempting to

remove the chain with his other hand, he received an

electric shock which so frightened him that he dropped

the flask. Musschenbroek, having tried the experiment,

I lie would not take a second shock for the crownof France.

The discovery created great excitement, and led to

the construction of improved instruments, to whichtht 1 name "Leyden jar"' was given.

The water in this instance constituted the inside

ting, the hand the outside coating: and, when the

other hand touched the chain, both surfaces were con-

nected by a conductor, and a discharge followed, which

produced the shock.

Fig. 20 represents the Leyden jar as it is usually

constructed. The essential elements are two conduct-

3 separated by a dielectric ; but, for conven-ience in charging and discharging, a wooden rap is

fitted to it. through which passes a brass rod, terminat-

ing in a hall above, and to its lower end is attached a


light Spring, or a chain, which comes into contact with

the inside coating.

Tin-foil is the usual coating, and is put on with paste,

covering both surfaces equally to within about three

inches of the top. Light sheet brass makes a more

substantial outside coating, and does not require at-

tachment to the surface.

It can also be used for

the inside coating, whenthe mouth is the full

width of the jar and the

sides are straight. Sul-

phuric acid is also some-

times used for the inside

coating of jars designed

for special purposes.

An instrument called

a discharger is also repre-

sented at A, in Fig. 20.

It consists of a curved

brass rod, terminating

in balls, and having an

insulating handle, of

ebonite or glass, at-

Fig. 20-Leyden Jar and Discharger. tached to its Center. It

is sometimes jointed at the center, and furnished with

two handles, as represented at B, Fig. 20. Its use is the

same as that of the bent wire already described.

The Leyden jar can be made of any insulating

material capable of being molded into the proper form

;

but glass seems to be the only substance capable of

resisting the enormous strain to which the dielectric is

subjected under a full charge.

ACCUMULA TORS. 11

Glass suitable for the purpose must be free from any

substance which makes it a partial conductor. Hence

such glass as is commonly used for fruit jars, candy

jars, and druggist's bottles cannot be used, since it con-

tains metallic substances.

Glass of a bright green color, free from bluish tint,

also the kind known as " hard flint," makes the best

Leyden jars.

The Leyclen jar is charged by an electric machine

;

its inner coating being connected with the machine!

and its outer coating with the earth, or with the op-

posite electrode of the machine ; though it is not

material which coating is connected with the machine,

except as a matter of convenience. The jar maybe insulated, and the charge given to the outer

coating, if the inner coating is connected with the

earth.

It is also immaterial whether the charge given is

positive or negative, as the opposite charge will always

be induced on the opposite surface ; electricity being

repelled to the earth when a positive charge is given,

or attracted from the earth when negative is given.

The electromotive force (E. M. F.) of a jar is equal

to the difference of potential between its inner and

outer coatings.

CHARGE BY CASCADE.—The method of charge by

cascade, first proposed by Franklin, is as follows: Let

a number of jars of equal size, as ^4, jB, C, D, be

arranged as represented in Fig. '21; the outer coating

of each, commencing with A, being connected with the

inner coating of the one next to it ; D having its outer

coating connected with the earth, and .1 having its

inner coating connected with the machine. And let A,


B, and C be well insulated on cakes of paraffin or some

equally good insulator.

A positive charge given to the inner coating of Awill induce negative on its outer coating, by repelling

the same amount of electricity ; and this repelled

charge must go to the inside of B, since it has no other

outlet. Hence the inner coating of B will be positively

charged, and electridiy will, in like manner, be repelled

from its outer coating to the inner of C. Hence the

charge of each jar in the series will be similar to that

of A ; electricity from the outer coating of B being

repelled to the earth.

Fig. 21—Jars in Cascade.

As the energy expended is distributed among four jars,

it is evident that the charge of each must be much less

than if the same amount had been expended in charging

one jar: since the energy accumulated cannot exceed

the energy expended. But, as the charge is in the

inverse ratio of the thickness of the glass, the resist-

ance from this source must increase from A to D, in

proportion to the number of thicknesses interposed:

and the charge must vary in the same ratio ; the. neg-

ative being greatest on the outer coating of A, where

only one thickness is interposed, and least on the outer

coating of D, where four thicknesses are interposed

;

ACCUMULATORS. 79

the positive on the inner coatings varying in the same

ratio. The same variation must also occur in the

resistance of the connectors, and produce a similar

effect, in a limited degree ; the resistance of a conductor

being directly as its length.

If the charge given to the inner coating of A be

negative, the electric movement is reversed; all the

inner coatings becoming negative, and the outer pos-

itive ; electricity being attracted from the earth to the

outer coating of D.

The insulations and connections should receive care-

ful attention, so as to prevent loss by leakage ; which

will inevitably occur if the insulation is imperfect, or

if the connectors have points, sharp edges, or projecting

corners.

After the charge is given, the jars should be sep-

arated, placed in connection with the earth, and each

discharged separately. A single jar, charged to the

same amount, should then be discharged, and the

results compared.

This method will indicate, roughhy, the amount of

charge of each jar ; but the electrometer, to be de-

scribed hereafter, will give more accurate results.

Tin: LiBYDBN Battery.—When a number of jars

have their inner coatings joined by conductors, andalso their outer coatings in like manner, the combi-

nation is called a Leyden battery.

A convenient form of such a, battery is represented

by Fig. 22, in which connectors between the inner

tin-' radiate from a central jar. The outer coatingsare made of sheet brass, nickel-plated, and screwed to a

wooden base, their connections being made with copperwires attached to the points of the screws underneath.


This construction for the outer coatings makes them

durable, gives the jars a firm attachment, and adds

greatly to the neatness and beauty of the instrument.

The E. M. F. of a Leyden battery is the same as that

of a single jar having the same amount of coated sur-

face. There can be no increase of intensity from any

special arrangement of the jars, as such a battery is

merely an accumulator, and not a generator of electric-

Fig. 22—Leyden Battery.

ity. But when great E. M. F. is required it is generally

more convenient to use a battery than a single jar of

equal energy. And, in case of fracture from an over-

charge, a small jar can be replaced at less expense than

a larger one.

In charging or discharging a battery, it is immaterial

which jar°is selected : for all the inner coatings being

connected together, as well as all the outer coatings,

each is practically the same as a single coating of equal

ACCUMULATORS. 81

size ; and connection with any part of either coating

affects the whole of that coating.

Discharge Through a Book.— The discharge

from a Lej^den jar or battery, passed through a card or

a thin book, leaves a puncture, with a burr projecting

from each surface.

Pig. 23—Discharge Through a Book.

To perform this experiment successfully, let oneknob of the discharger be placed in contact with the

outer coating, and the other in contact with the book;and let the book, held by its edge, with the knobagainst it, be brought quickly into contact with the

knob of the jar, and the discharge will take place as

shown in Fig. 23.


Iii this way a book of one hundred or more pages maybe perforated.

If the book is first placed in contact with the knob

of the jar, part of the charge will escape from the edges

and corners of the leaves, and the experiment is liable

to fail.

The burr projecting from each surface, after the dis-

charge through a book or card, has been relied on as a

proof of the dual nature of electricity, and ascribed to

the rush of positive and negatiye in opposite directions.

It is also attributed to the expansive force of heat, or of

gas, generated by the discharge.

The first theory cannot be accepted, unless we have

stronger proof of the dual nature of electricity than is

afforded by this experiment. And the second also fails

;

since in the case of a discharge through a book, the

leaves may be held so loosely as to allow a free outlet

for expansion from heat or gas, and yet the burr turns

in opposite directions from a point near the center of

the book, and becomes more prominent when the leaves

are thus held than when they are compressed ; whereas,

if the burr were due to the expansive force of confined

heat or gas, the reverse would be true.

Since these theories are unsatisfactory, let us en-

deavor to explain this phenomenon in accordance with

the principles which we have been considering.

Let a jar be charged on its inner coating, and

discharged through a book, as represented in Fig. 23.

Suppose the charge to be positive, electric movement

being from higher to lower potential, it would be from

the knob of the jar to the nearest knob of the dis-

charger. The entire charge of the inner coating,

passing out through the knob, would induce a high

ACCUMULATORS. 83

negative potential on that point, on the nearest surface

of the book, in a line between the knobs ; repelling the

electricity of the book along that line to the opposite

surface, which would thus become highly positive.

The paper being a very imperfect conductor, the

charges thus induced do not spread rapidly, but remain

concentrated for a moment on small circular spaces

around each of these points ; the greatest intensity

being at the centers. Hence there is a powerful at-

traction between the knob of the jar and this negative

point on the surface of the book ; and also between the

knob of the discharger and the positive point on the

other surface ; under the influence of which the paper

on each surface gives way and bursts outward toward

the knobs; that surface next the knob of the jar being

attracted, and that next the knob of the discharger

repelled.

As each outward leaf bursts, the next, becoming

then the outer one, bursts also, till the perforation is

complete from the center each way. All of which

occurs instantaneously.

Meantime the electricity from the knob of the jar

follows up this inductive effect on the electricity of the

book; but meeting great resistance from the imper-

fectly conducting paper, and the air between the leaves,

it is concentrated on each leaf successively; so that the

inductive force is constantly in advance of the charge,

the leaves and layers of air between them constituting

the dielectric.

It will be noticed, then, that tins is not a case of

energy going through a passive medium, but oienergyacting on the energy of that medium, causing it to becomeactive and perform zvork.


It should also be noticed that when the leaves are

held loosely, the thickness of the air dielectric is in-

creased ; each laj'er of air having a charged surface of

partly conducting paper on each side of it, is in the

position of the coated pane, a powerful attraction

between the surfaces acting across it. And when the

paper bursts there is more room for the formation of a

burr, and less resistance to the tearing of the paper,

which accounts for the increased prominence of the

burr.

If the charge of the jar is negative, the same results

occur in reverse order.

The Residual Charge.—When a Leyden jar is

discharged, there still remains a slight difference of

potential between the coatings, which is known as the

residual charge. Hence, a small discharge can be

obtained a moment after the first ; and this also leaves

a residual, bearing about the same proportion to the

second discharge as the second to the first, when the

same length of time elapses between them. A number

of successive discharges may thus be obtained, which

constantly decrease in amount till no further discharge

is perceptible. But, even then, it is not probable that

perfect equilibrium is restored.

To understand this, we must remember that even

the best dielectric is a partial conductor: and that

while electric movement is instantaneous in a good

conductor, it is very slow in a non-conductor. In the

Leyden jar we have a combination of both—two con-

ductors separated by a non-conductor. And, when the

charge is given, every part of each coating instantly

becomes electrified, one coating positively and the other

negatively, on the surfaces next the glass.

ACCUMULATORS. 85

The electricity, on the positively electrified coating,

slowly penetrates into the glass, acting inductively on

its electricity, which it repels from the opposite sur-

face ; and producing, probably, a temporary strain or

distortion of its structure.

When the first discharge takes place, there

is a relief from this strain ; and, as the

electrified glass slowly returns to its former

state, the electricity which had penetrated it

returns to the conducting surface.

This view receives confirmation from the

fact that delay increases the residual charge,

giving time for the electricity to come out of

the glass and accumulate : while it has the

opposite effect on the primary charge, reduc-

ing it by giving time for dissipation.

Tapping the jar lightly hastens the in-

crease of the residual charge, the vibratory

motion thus given to the glass tending, prob-

ably, to relieve the electric strain.

Jar with Movable Coatings.—If aJrithM^bfeLeyden jar be constructed with any rigid

Coatin» s -

metal, as sheet brass, for both coatings, as suggested onpage 76, and the conducting rod be attached to the

inner coating, the coatings may be removed and re-

placed at pleasure, as represented in Fig. 24 : and wehave the means of investigating certain phenomena in

regard to the electrification of the differenl parts.

Lei a charge be given to such a jar, and the coatings

removed carefully, so that they .-hall not be connectedby a conductor during removal: they may now he

brought into contact without producing any electric

effect : and the jar also may be handled with a like


result : but, on replacing the coatings, a full discharge can

be obtained, the same as if they had not been removed.

But if, while the coatings are removed, the jar be

examined by touching both surfaces with the finger and

thumb, or a small discharger, made with a bent wire,

at any point below a line marking the position of the

upper edges of the coatings, a discharge can be obtained

from that point. In this way a number of small dis-

charges can be had from various points, but no general

discharge.

This proves that the charge remains on the glass, while

the coatings are removed ; but that the resistance of the

glass prevents a general discharge. But it cannot be

accepted as proof that the charge is confined to the

glass, when the coatings are in contact with it ; unless

it can be shown that the charge remains on the glass

after the removal of both coatings at precisely the same

instant ; which could not be done with the care neces-

sary for so delicate an experiment. But when the

coatings are removed separately, the charge must be

transferred to the glass during the removal of each

:

since it is impossible to produce any change of poten-

tial on either surface, unless a corresponding change

is produced, at the same instant, on the opposite surface;

each being bound by the opposite.

Various Effects of the Discharge.—The dis-

charge of a Leyden jar of moderate size is sufficient to

explode gunpowder, and to ignite various substances

;

as phosphorus, powdered resin, sulphuric ether, and

alcohol; while that of a large Leyden battery fuses

wires, magnetizes steel, and destroys animal life.

With a battery of 550 square feet of coated surface,

large steel bars have been magnetized, iron wires, T|o

ACCUMULATORS. 87

of an inch in diameter, and 25 feet long, melted into

globules ; and tin wires, TV of an inch in diameter, and

8 inches long, dissipated in smoke.

Tyndall accidentally received a charge from a Ley-

den battery of " fifteen large jars " during a lecture,

and describes his experience as follows :" For a sensi-

ble interval life was absolutely blotted out, but there

was no trace of pain. After a little time consciousness

returned; I saw confusedly both the audience and the ap-

paratus. But though the intellectual consciousness of myposition returned with exceeding rapidity, it was not so

with the optical consciousness. For my body presented to

my eyes the appearance of a number of separate pieces.

The arms, for example, were detached from the trunk

and suspended in the air. In fact, memory and the

power of reasoning appeared to be complete long before

the restoration of the optic nerve to healthy action."

Gunpowder cannot be exploded by the ordinary

discharge; the only effect of which is to scatter it. But

when the discharge is retarded, by introducing into the

circuit an imperfect conductor, as a wet string, it

explodes readily. By this method also gun-cotton,

phosphorus, and other highly inflammable substances

may be ignited.

For such experiments the universal discliar</er, rep-

uted by Fig. 2-"), is convenient. It is constructed

with a base, in the center of which, mounted on a stem,

is a small circular tablet of some insulating material, as

ebonite; and at each end, mounted on insulating stems,

arc brass sliding rods, each terminating in balls, and

passing through a socket hinged on the top of its -tern.

A plaster of paris receptacle, to hold inflammable

substances, should also be provided.


The substance to be operated on is placed in thereceptacle on the tablet, the inner terminals of thesliding rods adjusted on opposite sides of it, and theouter terminal of one rod connected with the outercoating of the jar or battery ; and the circuit completedby connecting the outer terminal of the other rod with the

knob of the jar or battery, by the discharger, as shown.The wet string, or other imperfect conductor, when

used, can be intro-

duced into any con-

venient part of the

circuit, as at S.

Spontaneous Dis-

chaege.—A sponta-

neous discharge is

iable to occur in

attempting to charge

a jar beyond its ca-Fig. 25-Universal Discharger. parity : and, if the

glass is thin at any point, it may be fractured in this

way; but if the resistance of the insulating margin is

less than that of the glass, the discharge will take place

over that surface, without injury to the jar, electricity

always following the path of least resistance, whetherlonger or shorter.

Disruptive Discharge.—When a discharge takes

place through the air or any other dielectric, it is

termed disruptive; since the electricity must force a

passage and break down opposing barriers. Such a

discharge is always accompanied with light, heat, andsound; as expressed by the terms spark and snap, flash

and report—effects due to the resistance encountered,

and not qualities inherent in electricity.

ACCUMULATORS. 89

Silent Dischakge.—But when the discharge takes

place through a good conductor of sufficient size, it is

termed silent; since light and sound are absent; the

resistance encountered being only sufficient to produce

a slight amount of heat.

The discharge through a point is also termed silent

;

since a point, as already shown, offers no resistance

;

and hence there is little or no sound, even when the

discharge passes through intervening air. A battery

discharge, sufficient to destroy life, may be received

with impunity through the point of a cambric needle,

held in the hand, without producing any unpleasant

sensation.

Lichtenberg's Figures.—If, on a plate of ebonite,

or of glass varnished with shellac, figures be traced with

the knob of a positively charged Leyden jar, and sulphur

dusted over the surface, inclining the plate and tapping it

to remove the surplus; the sulphur will adhere to the

lines traced, spreading out in a beautiful fringe, as shown

in Fig. 26, which is from a photograph of a figure madein this way.

A similar result can be obtained by tracing lines

with the outside of this jar, or with the knob of a

negatively charged jar, and dusting the surface with

red lead.

( )r a mixture of sulphur and red lead may be used, and

separate figures traced; the jar being charged positively

for one figure, and negatively for the other. The sulphur,

it is claimed, adheres tojhe positive, and the lead to the

negative lines. Any non-conducting surface may be

used, also various other powdered substances.

It should be noticed thai the loss of charge, whether

positive or negative, from the inner coating, while tracing


the figures with the knob, is balanced by an equal loss

from the outer coating through the hand in which the jar

is held. Hence, when the tracing is made with the outer

coating, the knob must be held in the hand, to produce

the same effect on the inner coating : the jar being first

placed on an insulator to prevent a discharge and conse-

quent shock, by indirect connection through the earth.

Fig. 26—Lichtenberg's Figures.

An inspection of the figure shows, that at the point

where it begins above, the fringe lines radiate from a com-

mon center ; but that, as the curve is produced from right

to left downward and from left to right upward, they

point diagonally in the direction in which the knob of

the jar moves. The explanation is as follows :—The

ACCUMULATORS. 91

surface being a non-conductor, the electricity has to force

its way against strong resistance, bursting through at the

points where resistance is least, and forming the fringe.

The strongest effect is produced where the knob first

approaches the surface : as the jar has then a full charge

:

and the first action is a disruptive discharge through

the air, producing the circular, star-like figure, at that

point. But as the knob moves along the surface, after

contact, new lines start out at right angles to the line

of movement. And as the knob leaves a point where

such a line has started, it exerts an inductive action on

the original impulse, which tends to turn this line for-

ward; the diagonal direction being the resultant of

these two forces acting at right angles to each other.

And the forked branches are the result of similar

inductive action of the main fringe lines on the branch

lines.

We have, in this' experiment, a graphic demonstration

of the effect of an insulating surface in resisting electric

movement : since the figures show the exact location of

the electric force ; which, we see, is confined chiefly to

the tracings, spreading only to the limited extent

represented by the fringes.

It also shows that the effects produced in different

substances, by opposite electric influences, are depend-

ent on the electric condition of the substances them-

selves: so that a mixture or a compound may, in this

way, be separated into its elements. 'Flic sulphur in

this experiment becoming negative, as claimed, byfriction, is attracted to the positively charged lines,

while the red lead, becoming positive, is attracted to

those negatively charged. This principle has numer-ous useful applications in the arts.

CHAPTER VII.

Electric Generators.

The Electrophortts and Fricticxnal Machine.

The only electric generators noticed thus far are the

rods of glass, ebonite, and sealing-wax ; rubbed with

silk, woolen, or fur: but it is evident, that for such

work as the charging of Leyden jars and batteries, and

similar experiments, we require generators of far greater

capacity. But it was thought best to anticipate their

existence, and defer their introduction till there had

been a full consideration of the principles on which the

various kinds de-

pend: so that they

might all be in-

cluded in one com-

prehensive view

;

from which the

merits of each, andFig. 27-Eiectrophoms. the principles of

its construction could be more fully ascertained.

The Electrophorus.—This instrument, invented

by Volta, is one of the simplest forms of a static gen-

erator ; but it is of great utility in furnishing an

unfailing, though limited supply of electricity, for

numerous delicate experiments.

The following style, designed by the author, and

represented by Fig. 27, makes a handsome, convenient,

and very efficient instrument.

ELECTRIC GENERATORS. 93

On a wooden base thirteen inches square, constructed

of layers glued together to prevent warping, is placed

a thin sheet of brass of the same size ; over which is

placed a sheet of ebonite of equal size, TV of an inch

thick ; and both attached to the base by screws near

the corners.

On the ebonite is placed a circular plate or cover,

made of No. 20 sheet brass, twelve inches in diameter,

perfectly flat, and having a round resistance rim joined

to the upper surface. In its center is an ebonite handle,

seven inches high ; and from its rim projects a goose-

neck, made of No. 8 brass rod, terminating in a half-

inch brass ball ; near which, on the edge of the base, is

a brass strip, § of an inch wide, connected with the

lower plate.

The base may be made of metal, if preferred, in which

case the lower plate and strip are unnecessary, the base

itself taking the place of the plate.

In this instrument we have two conductors separated

by a dielectric ; the upper one insulated, and the lower

connected with the earth.

The cover being removed, the dielectric is beaten

briskly with a piece of catskin, or other fur, by which

its upper surface is electrified ; and the cover is then

replaced.

Suppose the charge to be negative ; electricity having

been removed by the fur, the same amount is attracted

from the earth to the under surface of the dielectric,

and to the upper surface of the brass plate in connection

witli it : which thus become positive by induction. The

under surface of the cover also becomes positive and its

upper surface negative.

Let a connection now be made between the lower


plate and cover, by touching the strip and knob with

the finger and thumb, or a small discharger ; the elec-

tricity accumulated on the lower plate will pass to the

cover, producing a shock if passed through the hand.

The cover thus becomes positive ; but its charge is

neutralized, or bound, by the negative of the dielectric.

Let it be lifted off by the insulating handle ; its charge

being no longer bound, a discharge, producing a spark,

an inch or more in

length, takes place,

when the knuckle

or any conductor

is presented to the

knob, as shown in

Fig. 28.

The removal of

the cover with its

positive charge,

having left the up-

per surface of the

dielectric negative,

a positive charge

is again attracted

to the under sur-

face and plate, as before ; and the cover, having been

discharged and replaced, the process may be repeated

with the same results an indefinite number of times,

and Leyclen jars charged, or other electric work

performed.

Suppose the original charge to be positive, the same

results occur in reverse order. Electricity having been

imparted by the fur to the upper surface of the dielec-

tric, the same amount is repelled from the under

Fig. 28—Discharge of Electrophorus.


surface and plate, making them negative. The under

surface of the cover also becomes negative and its

upper surface positive. Connection being made as

before, electricity passes from the cover to the lower

plate ; leaving the cover negative, and its charge bound

by the positive on the upper surface of the dielectric.

The cover being removed, and a conductor presented

to the knob, a discharge takes place ; electricity nowpassing from the conductor to the cover, instead of from

the cover to the conductor as before.

The removal of the cover, with its negative charge,

having left the upper surface of the dielectric positive,

electricity is again repelled from the under surface and

plate by induction : and the cover having been restored

to zero and replaced, the process may be repeated as

before.

We see. then, that when the charge is negative,

electricity is attracted from the earth to the lower

plate, then passes to the cover, and then from the cover

to the presented conductor; but when the charge is

positive, electricity is repelled to the earth from the

lower plate ; then an equal amount passes from the

cover to the lower plate, and the same amount passes

to the cover from the presented conductor.

Hence, when the instrument receives a positive charge,

it gives a negative charge; and when it 8 a neg-

ative charge, it gives a positive charge.

It will also be noticed that the initial charge is given

by friction, but all subsequent charges are obtained by

induction.

If the cover be removed, without first making con-

nection between it and the lower plate, no charge will

be found on it: since it lias neither gained nor lost


electricity through any external source ; and its ownelectricity, being merely changed to the upper or lower

surface, by the positive or negative of the dielectric, is

restored to zero when removed from that influence.

This connection may be made automatically by plac-

ing a short brass pin in a hole made through the dielec-

tric, its upper end even with the upper surface, so that it

shall touch the cover and also the lower plate. This

makes the instrument more convenient for obtaining

charges in rapid succession: but, when used to demon-

strate the principles involved in its construction, as

above, the pin should be removed.

The top of the handle should be grasped, whenremoving the cover, to prevent a partial discharge

through the hand.

The electrophorus will retain its charge for months

;

and, like the Le} rden jar with movable coatings, can be

taken apart and put together again without perceptible

loss of charge ; but, when not in use, the charge is

gradually dissipated, so that only a residual remains.

Hence it should be charged again before immediate

use, if great efficiency is desired. This property of

constancy probably suggests the name, electrophorus,

electricity-bearer, from (jpeooo to bear, r\lvA.xqov electricity.

The Frictioxal Machine.—The principle of this

machine is the same as that of the rod and rubber. It

was invented b}^ Otto Guericke, and consisted, at first,

of a globe of sulphur, revolved on an axis by a crank,

the hand being used as a rubber. Subsequently a globe

of glass was substituted for the sulphur ; but as insu-

lation was disregarded in both styles, only feeble results

were obtained, and the machines fell into disuse.

Boze, of Wittemberg, revived and improved them,


using the glass globe, and a band wheel and belt to

increase their speed ; and collecting the electricity on

an iron tube, suspended by silk cords, from which hunga chain in contact with the globe.

Further improvement was made by the use of a

leather rubber stuffed with hair : and subsequently the

globe was replaced by a glass cylinder, on one side of

Fig. 29—Plate Electrical Machine.

which the rubber was mounted on a glass pillar; andon the other side, similarly mounted, was a brass cylin-

der, called the prime conductor, from which a row of

points projected toward the glass. An oil silk flap

enveloped the upper part of the glass cylinder; and a

chain was used to connect either the rubber or theprime conductor with the earth, as desired.

The plate machine, invented about 1787, was con-


structed on the same principles, a glass plate being

substituted for the glass cylinder, and has now come

into general use. Fig. 29 represents one of the prevail-

ing styles.

It consists of a disc of plate-glass A, mounted on a

wooden base with wooden or glass pillars, and revolved

by a crank with an insulated handle. A pair of rub-

bers i?, made of soft leather or felt, are pressed against

the glass on opposite sides by a pair of brass springs (7,

the pressure being adjusted by a screw. These are

mounted on a glass pillar, and connected above with a

brass ball ; and a brass chain, which may be removed,

connects them with the earth.

Mounted on a glass pillar is the prime conductor D,

made of brass, and consisting of a pair of balls, from the

lower one of which projects a pair of combs, which

extend on opposite sides of the glass, and whose teeth

come within a quarter of an inch of it. And, from the

opposite side of the same ball extends a rod, terminat-

ing in a small ball.

A silk cover envelops the lower part of the glass plate,

and the rubbers, on the surfaces in contact vith the glass,

are coated with an amalgam, composed of five parts

zinc, three parts tin, and nine parts mercury, melted

together, pulverized, and made into a paste with lard.

The machine should be dry and warm before use, as

moisture condenses on the surface of the glass when it

is colder than the atmosphere, and suspends insulation.

For this reason ebonite pillars have an advantage over

glass, being less liable to condense moisture.

Ebonite has also been used for the plate, but is not

so reliable as glass ; and its liability to warp with heat,

when in thin plates, makes it very objectionable.


Its Mode of Actiox.—The plate being revolved in

the direction of the arrow, electricity is generated by

the friction of the rubbers; the charged surface of the

glass passing directly into the silk cover, which prevents

loss of charge from contact with the air.

If the charge on the glass is positive, when the

charged surface comes opposite the combs, electricity

passes through them from the plate to the prime con-

ductor, where it accumulates. The glass, being thus

discharged, passes round again to the rubbers, which,

having become negative from parting with electricity

to the glass, have received electricity from the earth

through the chain.

Each portion of the plate is thus alternately charged

and discharged, as it passes first to the rubbers, and

then to the combs ; the lower half being constantly

positive, and the upper half at zero, except the resid-

ual ; electricity passing to the rubbers from the earth,

and being carried round by the plate to the prime

conductor.

If the charge on the plate is negative, the transfer

takes place in reverse order ; electricity passing from

the prime conductor to the plate, from the plate to the

rubbers, and from the rubbers to the earth ; the prime

conductor becoming negative and the rubbers positive.

If the prime conductor be placed in connection with

the earth, by having the chain transferred to it, the

charge, whether positive or negative, will take place on

the ball and other parts connected with the rubbers.

If the prime conductor and rubber be connected bythe chain, no charge can occur on cither: since elec-

tricity constantly passes from one to the other through

the chain, as it is generated.


If the chain be removed entirety, only a veiy limited

charge can occur, derived from the material of the

machine itself.

The limit of the charge is reached when its potential

energy, whether positive or negative, so far exceeds the

resistance of the air, that the loss of charge by convec-

tion, as explained on page 66, shall equal the energy

generated. When the atmosphere is damp, or its

electric potential low, this limit is soon reached; but

when dry, and at a high electric potential, a muchgreater charge can take place.

Machine Described by Noad.—The largest ma-

chine of this kind of which we have any record was

made some years ago for the Panopticon of Science in

London. According to Noad, it had a plate ten feet in

diameter, three pairs of rubbers, each three feet in

length, and a pear-shaped prime conductor, six feet

in length, and four feet in diameter at its widest

part-

It was operated by steam power, and gave sparks

fifteen to eighteen inches in length; and charged to its

full capacity, in less than a minute, a LejTden battery

of thirty-six jars, having one hundred and eight square

feet of coated surface.

Measurement of Energy.—The amount of elec-

tricity which a well-constructed machine can generate

is in proportion to the surface area of the plate, which

may be increased to any practicable limit, the other

parts being increased in like proportion. It is roughly

estimated by the number of sparks of a given length

and energy which can be obtained in a given time,

when an uninsulated conductor is brought near the

prime conductor; or by the length of time required to


charge a Leyden jar or battery having a given amountof coated surface.

The results are only approximate, especially those

by the first method, for the following reasons. Lengthof spark is not a true index of energy ; since a short,

thick spark may have greater energy than a long, thin

one: and our estimate of the comparative energy of

each from its appearance, and the accompanying snap,

is liable to be very inaccurate. The spark accom-

Fig. 30—Lane's Unit Jar.

panying the discharge of a Leyden jar or battery is

generally quite short, though its energy often greatly

exceeds that of any single spark of much greater length,

given by the machine in charging it.

The humidity of the air and its electric potential

being liable to great variation, produce a correspondingvariation in the results obtained at different times.

The charge and discharge of a Leyden jar of a givencapacity, in a given time, is a more reliable method.


The jar should be made self-discharging, by bringing

the knob of a conductor, connected with its outer

coating, within sparking distance of the knob of the

jar.

Lane's unit jar, shown in Fig. 30, is constructed on

this principle.

A bent brass rod is connected by a band to the outer

coating; its upper end terminating in a ball through

which passes a horizontal sliding rod, terminating in a

ball at its inner extremity; and having an ebonite

handle at its outer extremity, by which the ball can

be adjusted to any required

distance from the knob of the

jar.

To estimate the comparative

energy of different machines,

a uniform rotation of the plates

must be maintained by a given

number of revolutions per min-

ute; and the number of dis-

charges in a given time of theFig. 31—Electric Chime. • , • , ^ •,-! ,i& unit jar, connected with the

prime conductor, will then be approximately correct

for the energy of each.

The Electric Chime.—This instrument is used in

connection with the machine, to illustrate electric

attraction and repulsion.

It may be mounted on a separate stand, or hung from

the projecting rod of the prime conductor. Fig. 31

represents a common style used in this way.

It consists of three bells suspended from a brass rod;

the two outer ones by brass wires or chains, and the

central one by a silk cord; a brass chain connecting it


with the earth. Between the central and outer bells

are two small brass balls, suspended by silk cords.

When the machine is put in operation, the outer bells

receive a charge from the prime conductor; this acts

inductively on the insulated balls, which are at zero,

attracts, and, after contact, repels them. Being now

charged the same as the outer bells, they act inductively

on the central bell, repelling or attracting electricity

through its chain, according as their charge is positive

or negative ; and pro-

ducing on it a charge

of the opposite kind,

they are attracted to it

and discharged. Be-

ing now at zero, they

are attracted to the

outer bells, as before ;

and in this way the

three bells are madeto ring.

Image Plates.—These are used to

show the effect of in- FiS- 32-Iinage Plates.

duction between two conducting surfaces, as repre-

sented by Fig. 32.

From the projecting rod of the prime conductor, a

brass plate, having a resistance rim, is suspended by a

wire or chain: and under it. on an insulating stand, is

placed another similar plate, made a little larger, andjoined to the insulating support by a sliding rod, bywhich the distance between the plates may be adjusted,

a chain connecting it with the earth.

When the machine is put in operation, the upper


plate will have the same charge as the prime conductor.

If the charge be positive, electricity is repelled by in-

duction from the lower plate to the earth, through the

chain ; if negative, it is attracted through the same

medium ; and, in either case, the plates are oppositely

charged to the same potential, the air being the dielec-

tric.

When the space is properly adjusted, pith balls or

images, placed on the lower plate, *are alternately at-

tracted and repelled, dancing up and down between

the plates in a manner which is often quite amusing.

If electric connection

with the earth be sev-

ered by removing the

chain, this effect will

cease : which proves

that the opposite poten-

tials of the plates was

caused by the transfer

-The El^tric Whirl. of electricity to or from

the earth, as stated.

The Electric Whirl.—This little instrument,

shown in Fig. 33, consists of a set of pointed brass arms

attached to a common center, which is pivoted on the

point of a vertical rod connected with the prime con-

ductor; the arms being bent, so that when passing a

given point each shall turn in the same direction.

When the machine is put in operation, the air in

front of each point becomes electrified, either positively

or negatively, by the passage of electricity either from

or to the point; while that back of it is oppositely

electrified by induction. This causes repulsion from

the air in front, and attraction toward that at the back,


producing rotation of the instrument in the opposite

direction to that in which the points turn.

The effect of a stationary point in producing a cur-

rent of air is shown in Fig. 34; where the flame of a

candle is represented as blown from a point attached

to the prime conductor.

The direction of the air current will be the same

whether the charge is positive or negative : since, in

either case, the air embraced within a sphere of which

the point is the center will have the same potential as

the prime conductor; while that outside of this sphere

will assume the opposite potential by induction. Hencethe air near the

point becomes

self-repellent,

and is also at-

tracted by the

air outside ; that

directly in front

of the point

being repelled

with the greatest force, produces a current in that di-

rection, while the air on either side is attracted, and, in

its turn, again repelled.

Armstrong's Hydro -Electric Machine.— Fig.

35 represents a machine invented by Sir William Arm-strong, about 1840, which generates electricity by the

discharge of partially condensed steam.

It consists of a boiler and furnace mounted on glass

pillars; the boiler being provided with steam and water

gauges, a safety valve, and a condenser inclosing sev-

eral small pipes, through which the steam escapes.

These pipes are surrounded with filaments of cotton,

4—Air Current from a Point.


the lower ends of which are immersed in cold water at

the bottom of the condenser: and the water being thus

raised by capillary attraction, cools the pipes, producing

partial condensation of the steam ; thus charging it

with water in fine drops, by the friction of which

against the pipes electricity is generated ; the steam

Fig. 35—Armstrong's Hydro-Electric Machine.

being discharged against a row of points connected

with the prime conductor.

Each pipe is furnished with a wooden tip : and the

friction is increased by a tongue of metal, around which

the steam must pass before entering the tip, as shown

by the enlarged section at letter A.

A machine of this kind, constructed for the Roj'al


Polytechnic Institution in London, had a boiler seventy-

eight inches long, and forty-two inches in diameter, with

forty-six steam jets. It gave sparks twenty-two inches

in length, and charged to its full capacity, in six to eight

seconds, a Leyden battery, having eighty square feet of

coated surface.

Another one, described by Noacl, had one hundred

and forty steam jets, gave sparks of the same length,

with three or four times the rapidity; and charged, to

its full capacity, a Leyden battery having 1,188 square

feet of coated surface, sixty times in a minute.

But though capable of such powerful effects, this

machine is not practical. It is inconvenient to manage,

requires distilled water, careful cleansing of the boiler

after use, and great steam pressure. Its operation is

accompanied with a deafening noise, and the escape of

a great volume of steam, producing dampness and other

unpleasant results, when used in a room. Hence its

chief value is in the demonstration of the important

fact, that electricity may be generated in this way.

CHAPTER VIII.

Electric Generators.

The Holtz and Topler Machines.

Influence Machines.—Previous to 1865, frictional

machines were the principal electro - static generators

in use. But.that year marked an era in electric prog-

ress by the invention of two new machines of remark-

able energy, by the German electricians, Holtz and

Topler; to which the name influence machines was

given, from their being constructed with two or more

glass plates, arranged to generate electricity by their

mutual inductive influence.

Both machines are very similar in construction ; the

principal difference being, tjiat the Holtz requires to be

incited by an initial charge from an external source,

while the Topler is self-inciting.

The Holtz Machine.—This machine, of which

there are several different styles, is represented by Fig.

36. On a wooden base are mounted two glass plates ;

the rear plate B stationary, and supported by three

ebonite insulators, two below and one above ; while the

front plate A revolves in the direction of the arrow,

on a steel shaft, which passes through an opening in

the center of the plate J?, and is attached to the post

at M. A is mounted on an ebonite hub, attached to a

hollow shaft of brass, which revolves on the fixed shaft,

and carries, at the end next the post, a small pulley,

from which a belt extends to the driving wheel, which


is revolved by a crank with an ebonite handle. The

relative sizes of the wheel and pulley are such as to

give the plate four to six revolutions for each revolu-

tion of the driving wheel, the plates of small ma-

chines requiring a more rapid revolution than those of

larger ones. In front of the plate A, £ of an inch from

the glass, are the combs "Fand H, attached to a brass

core at the center of the ebonite disc M; and the

combs iT and I>, insulated by their attachment to ebon-

Fig. 3G—The Holtz Electric Machine.

ite rods projecting from the disc ill", and connected by

brass rods with the Leyden jars C and D, and with the

sliding-rods P and R. These sliding-rods have ebonite

handles, and terminate in brass balls at their inner ex-

tremities.

The plates are of sheet glass, about \ of an inch thick;

of good insulating quality, and well coated with shellac.

The stationary plate i?has two circular openings called


windows, directly opposite the combs iT and L ; and, on

its rear surface, are cemented two paper inductors Tand X; T extending from H to i, and X from V to K;and each armed with a row of points, projecting into

each window.

Machines of this kind are often constructed with

more than two plates ; sometimes with a large number.

The plates are also sometimes placed in a horizontal

position. Ebonite plates are also used ; but are objec-

tionable, for reasons already given.

The Topler Machine.—The Topler machine has

the same general construction as the Holtz ; but, on

the front surface of the revolving plate, are cemented

a number of small metal discs, called carriers; usually

made of tin-foil with raised brass centers, which, as the

plate revolves, are brought into contact with four wire

brushes ; two attached to the stationary plate, and two

to the uninsulated combs. In this way the machine is

made self-inciting, as already mentioned.

The windows, and the rows of points projecting into

them, used in the Holtz stationary plate, are omitted

from the stationaiy plate of the Topler: and the paper

inductors are made longer, and have small tin-foil in-

ductors under them, connected, by tin-foil strips, with

each other and also with the two brushes attached to

this plate.

Fig. 37 represents a Topler machine constructed by

the author, and patented April 10, 1883, and December

8,1885. The principal points covered by the patents

are as follows :—

1. The outside coatings of the Leyden jars C and

D are of sheet brass, nickel plated ; and are screwed

firmly to the base ; forming cups into which the jars

ELECTRIC GENERATORS. Ill

fit closely, and are thus held in a fixed position ; afford-

ing a firm support to the parts connected with them,

and preventing liability to accident or injury to the jars

or plates.

2. The induced current from these outside coatings

is conveyed down by the brass screws which attach

Fig. 37—Atkinson's Topler Electric Machine.

them, and along copper wires underneath, to the termi-nals of the switch S ; through which, when closed, it

passes from one jar to the other ; but when open, as in

the cut, it passes by the brass sockets, seen on the edge,which are also connected with the terminals, outthrough the conducting cords, and a person, or other

112 ELEMENTS OF STATIC ELECTRICITY,

object, connected with their outer extremities. As this

induced current flows simultaneously with the direct

current from the inside coatings, the switch and sliding-

rocls place it completely under control of the operator.

3. The brush holders, U and F, are attached to the

plate B, through holes near its edge ; tlius giving a di-

rect passage to the electricity from the carriers on the

plate A, where it is generated, through the glass, to the

tin-foil inductors, represented by the dark shade, and

the paper inductors T and X, represented by the light

shade. By passing the electric charge through the glass,

inside its edge, an insulating margin is interposed be-

tween the conductors and the edge, thus preventing

loss from leakage, which is unavoidable when the brush

holders are attached by clamps or ears on the edge.

4. The carriers on the plate A are of sheet brass,

with raised centers, and are nickel plated, making them

both durable and ornamental. The hard nickel surface

is not affected by the action of the brushes, or the elec-

tricity, while tin-foil soon becomes defaced : and the

carrier, being practically one piece, and its entire sur-

face cemented to the glass, its raised center cannot be-

come detached, as may happen when the center is put

on separately over a tin-foil base.

5. The combs V and K, also H and i, radiate at an

angle of 45 degrees to each other, from the central disc

M, to which they are attached ; so that any possibility

of error in regard to their position, or of displacement, is

practically impossible.

The following improvements may also be noticed:—The base is made of two-inch strips, glued together

lengthways, and heavy cleats screwed on underneath

;

giving all the advantages of iron as to freedom from


warping, with the insulation and elegant finish of the

wood. The driving wheel is of ebonite and the iron

casting, on which it is mounted, slides in grooves on an

iron plate, and is moved by the adjusting screw 0, to

regulate the tension of the belt.

Fig. 38—Atkinson's Four-Plate Topler Machine—Front View.

The ebonite insulators,which support the plate B, have

soft rubber packing, to ease the pressure on the glass.

The conducting rods of the Leyden jars pass through

ebonite caps with cork attached underneath, which

gives them a fixed vertical position, and affords firm

support to the sliding-rods and the combs connected

with them above.


The Four-Plate Topler Machine.—This machine

has the same construction in front as the two-plate ma-

chine as shown by Fig. 38, but a special construction

for the two rear plates which will be understood by

reference to Figs. 39 and 40.

The end view, Fig. 39, shows two pairs of plates, the

position of the rear pair being reversed, which brings

the stationary plates into the center, back to back, be-

tween the revolving plates

;

so that the inductors are on

the inner surfaces of the

stationary plates, and the

carriers on the outer sur-

faces of the revolving plates,

which being mounted on

the same shaft, with a col-

lar between them, revolve

in unison.

The combs L and K, and

T^and H, have curved rods

which pass round the

plates and support dupli-

cate combs in the rear as

shown in the cuts. The

brushes are also duplicated

as shown : so that with the exception of the Leyden

jars and switch, and parts connected with them, this

is practically a double machine.

In like manner an eight-plate machine may be made

by doubling these parts of the four-plate.

When the large Topler or Holtz machines are

wanted for constant use, the motive power is usually

supplied by a steam or gas engine, or a water motor.

Fig. 39—Atkinson's Four-Plate

Topler Machine—End View.


In which case the driving wheel is not used ; the belt

passing directly from the small pulley connected with

the plates, to a pulley attached to the engine or motor.

Mode of Action of the Toplep.—To compre-

hend the action of any electric generator, the following

essential principles in their construction should be kept

distinctly in view.

To generate electricity, is to create a difference of

electric potential ; the efficiency of all generators,

Fig. 40—Atkinson's Four-Plate Topler Machine—Rear View.

whether batteries, dynamos, or glass plate machines,

depending on the difference of potential which each is

able to create and maintain within the apparatus itself.

And the work to be done by such an apparatus is the

restoration of equilibrium, through an exterior circuit

:

and may consist in producing heat or light, chemical,

mechanical, or phj^siological action.

Let us consider how these principles apply to this

machine.


Fig. 37, page 111, represents the machine with the

sliding electrodes P and R separated. Suppose the

switch & to be closed and the machine put in operation.

It will be seen that as the plate A revolves, the raised

centers of the six carriers are brought into contact with

the wire brushes attached to the holders E and F; each

opposite pair touching opposite brushes, successively,

at the same instant. The friction generates electricity,

which diffuses itself over the carriers on A, and the in-

ductors on B, with which they are, at the instant of

contact, in electric connection. The potential of car-

rier and inductor, during contact, will be the same : at

the next instant the carrier passes on, and is insulated

from the inductor, and carrier and inductor now act in-

ductively on each other, and multiply the initial charge

given by the friction of contact. As it accumulates, it

spreads over the paper inductors ; these act on the

opposite surfaces of the glass, till botli surfaces of both

plates become charged ; the initial charge being still

continued by the constant friction of the carriers and

brushes. •

But, since both sides of the machine are of similar

construction, and since the mode of action on both

sides is apparently the same, the question arises, how

any difference of potential, or electric charge can be

accounted for.

And first, it will be noticed, that the position of the

plates being vertical, their lower halves are nearer to

the earth, by their semi diameter, than the upper halves,

and consequently, more under the influence of its in-

ductive action, by the square of that distance. The

lower halves are also in close proximity to the Leyclen

jars, the driving wheel, and the belt, and subject to their


inductive influence; and the plate B is supported on two

insulators, while the upper half has but one, and hence

has the advantage of the better insulation of the air.

To this lower half of B, and subject to these influ-

ences, is attached the brush holder F, while B is

attached to the upper half, and remote from them.

Hence, the carriers brushed by E, and descending to-

wards i, must acquire a higher potential than those

brushed by F, and ascending towards K.

An accumulation of electricity must also occur at the

lower ends of the inductors Tand X, from the induct-

ive influence of the earth ; and as the brush holder Fis placed at the lower end of X, it furnishes an outlet

to a portion of this charge, as seen at night by the

brushes of light from this holder to the outside of the

jar C\ and other parts in close proximity.

The lower end of T, on the contrary, is well insu-

lated ; hence the potential of T7

, from the heavier charge

at its upper end, and the better insulation at its lower

end, must be much higher than that of X, where the

influences are just the reverse.

This accumulation, or high positive potential at the

lower end of 7, produces a high negative potential at

that point on the plate A, and its carriers, as it revolves;

as shown by the brush of light, seen in the dark, from

the uninsulated comb J7, marking the flow of electricity

to the upper part of the plate, as it passes under that

comb; the outflow of the current received through the

comb II This brush of light extends downward, as

the charge increases, almost to the comb K: and a sim-

ilar brush extends downward from K, marking the

outflow of electricity from the interior of the jar (7, as

explained hereafter: while the points of the combs, B


and H, where the charge is received, show only a glowof light.

These brushes of light always turn in the opposite

direction to that in which the plate A revolves ; differ-

ence of potential between the comb and that portion of

the plate approaching it producing attraction ; while

equality of potential between the comb and that portion

of the plate receding from it produces repulsion, (seep. 224.)

Following any opposite pair of carriers, as IF and Z, wefind that as Z passes under the wire brush i7

, W passes

under E ; and as Z moves on to the insulated comb if,

IT at the same instant arrives at L ; but W, as already

shown, has a higher potential than Z, and, at this point,

a peculiar adjustment takes place. W gives up its

charge through the comb i, to the inside of the Ley-

den jar D. This creates a positive charge on the inside

of D, which induces a negative charge on its outside.

The electricity thus repelled, passes to the outside of

(7, making it positive, and inducing negative on its

inside ; and this repelled electricity flows through the

comb K to the plate ^4, as already shown. W then

moves down to the uninsulated comb ZT, while Z moves

up to V. Each now passes under the wire brush at-

tached to its respective comb, and the combs being

attached to the brass core at the center of iff, the carriers

are put in electric connection with each other, and their

potential equalized by the flow of the residual charge

from II to J7", as already described; so* that each arrives

at the original position of the other at the same poten-

tial, ready to repeat the same process.

It should be noticed, that the residual is slightly in-

creased by induction from T and X, as the carriers move

from the combs L and if to the combs iTand V.


The surfaces of the p]ates, on which the carriers and

inductors are mounted, assume the same potential as

the carriers and inductors attached to them, while their

opposite surfaces have the reverse. Opposite parts of

the same surface are also in opposite electric states : the

section L M H, for instance, having a potential oppo-

site to that of VM K; change of potential on these

surfaces following that of the carriers and inductors,

already described.

It will be noticed that the office of the brushes, IE and

_F, is the reverse of that of if and V. U and F generate

by friction, while H and V discharge by contact. And,

while the combs, K and Z, aid in creating a difference

of potential, the combs, H and J7", aid in restoring equi-

librium.

When the difference of potential between the inner

coatings of the jars becomes sufficient to overcome the

resistance of the air, a discharge from the inner coating

of D to that of C takes place between the terminals of

the sliding electrodes R and P ; and, at the same

instant, q, discharge from the outer coatings takes place

through the switch and connections, from C to D, to

restore equilibrium between them, and thus complete

the circuit.

A spark and snap, from the resistance of the air,

accompanies the discharge between the inner coatings

;

and the same will occur between the outer coatings if

the switch is open ; but, if closed, the discharge takes

place silently. The plates and other parts being, at the

same instant, relieved of strain, there is a restoration of

equilibrium in the whole machine.

The above explanation applies to the machine whenit is put in operation from a state of absolute rest ; but


when it is in a high state of activity, there frequently

occurs a reversal of potential after a discharge, as shownby the reversal of the brushes of light from the combs.

To account for this it must be considered, that the

residual which remains after the primary discharge may,

from unequal resistance, be greater on one side than on

the other: and after being relieved from strain by the

primary discharge, it will operate to give a slight pre-

ponderance of potential to that side, which is rapidly

multiplied by induction, as the rotation of the plate

continues.

A reversal can also be produced by a temporary

reversal of rotation, as explained on page 1-10 ; or by

touching the inductors, or parts connected with them,

while in action, which would reduce the potential at that

point. Special conditions may also exist in certain

machines, which will reverse the ordinary mode of

action ; as, for instance, a difference of thickness on

opposite parts of a glass plate ; or in opposite jars.

It should be noticed that the electric charge is

instantly diffused over the metal carriers and inductors,

more slowly over the paper inductors, and still more

slowly over the shellacked surfaces of the glass plates.

So that when the machine is put in action, after a con-

siderable interval of rest, three or four seconds elapse

before it becomes fully charged, and a crackling sound

is heard from the electricity forcing itself over the

resisting surfaces of the paper and glass.

The condition of the air, as to its insulation, influ-

ences the whole operation of this machine. An air

space insulates the plates, and also the jars, with their

rods and balls, from each other; and as a damp atmos-

phere lessens this insulation, it will decrease the energy


of the machine in like proportion. A film of moisture,

settling on the plates, will often so reduce the insula-

tion, that the slight initial charge bj the action of the

brushes is conducted over the damp surface as fast as

it is generated ; so that no difference of potential, and

consequently no permanent charge, can occur. And as

the machine is much more sensitive to such influences

than the operator, the latter is often puzzled to knowwhy it will not generate. The simple and effectual

remedy, in all such cases, is to dry it. This may be

done by a fire, a kerosene lamp, a hot iron, or by

the sun's heat, though artificial heat is generally more

effectual.

Warm days, before or after rain, when the atmos-

phere is loaded with moisture, are the most unfavorable.

At such times the plates should not only be dried, but

warmed, as moisture will continue to be deposited so

long as they are colder than the air.

The electric conditions in upper rooms, other things

being equal, are more favorable to the operation of the

machine than in those on the ground floor.

Multiplication of the Chaege.—The multipli-

cation of the initial charge proceeds with great rapidity.

During the first revolution of the plate A, each tin-foil

inductor receives six direct charges from the contact of

its connecting brush with each of the six carriers: and

also six inductive charges of equal amount, as each

charged carrier passes it. So that at the end of the

first revolution, it has accumulated twelve charges ; and,

during that revolution, it has reacted inductively on each

passing carrier with this constantly increasing energy,

increasing the energy of the carrier in like proportion.

At the beginning of the second revolution, it has


twelve times the inductive energy which it had at the

beginning of the first ; and this energy continues to

increase, and react op the carriers, at the same rate as

before. And as the plate makes about five revolutions

per second, the rate of increase on any tin-foil inductor

is about sixty increments per second.

But as the charge spreads from the tin-foil inductors

over the paper inductors and adjacent parts of the sta-

tionary plate; and from the carriers over adjacent parts

of the revolving plate, each point on each plate, within

the charged areas, becomes a center of direct and induct-

ive action in the same manner as the metal inductors and

carriers. So that even an infinitesimal charge is increased

in a few seconds to the full capacity of the machine.

HOLTZ AND ToPLER* MACHINES COMPARED.—Since

the chief difference between the Holtz and Topler con-

sists in the latter being self-inciting, the mode of action

is essentially the same in each.

The" Holtz may receive its initial charge from a fric-

tional machine, an electrophorus, or any similar, exter-

nal source : but the usual method of charging is by

means of a piece of ebonite, electrified by the fur of a

cat-skin.

The electrified ebonite is held in contact with one of

the paper inductors on the stationary plate, which is

thus charged ; a portion of the charge being commu-

nicated to the revolving plate through the points which

project into the windows ; and this plate is made to

rotate rapidly, so that the charge is soon multiplied to

the full capacity of the machine, if the atmospheric con-

ditions are favorable; and the ebonite is then removed.

It will thus be seen that the initial charge in both

machines is produced by friction and multiplied by


induction. In the Holtz it is derived from an external

source, begins on the stationary plate, and is then com-

municated to the revolving plate. In the Topler it is

produced by the machine itself, begins on the revolving

plate and is then communicated to the stationary plate.

In the Holtz it occurs on one side only. In the Topler it

is simultaneous on both sides. In the Holtz it ceases when

the plates are charged. In the Topler it is continuous.

The absence of the brushes, carriers, and metal in-

ductors from the Holtz increases the internal resist-

ance, making it more difficult to charge, but giving

better insulation, and consequently greater energy than

a Topler of the same size.

But the action of a Holtz is much more liable to

interruption from dampness, and a low electric poten-

tial in the atmosphere : since it receives only a small

initial charge, which is soon discontinued ; while that

of the Topler is constant, from the continuous action

of the carriers and brushes. So that a well constructed

Topler, with ordinary care, is reliable in any state of

the atmosphere, while a Holtz is very unreliable.

Comparison by Dr. Holtz.—In reply to an

inquiry as to whether the Topler machine was an

original, independent invention, or only a modification

of the Holtz, the author received a letter from Dr.

Holtz, written from Greifswald, Germany, March 20,

1883; in which he says, that his machine, as first de-

scribed in Poggendorffs Annalen, in 1865 (volume

125, page 469, and volume 126, page 157), had "twodiscs rotating in opposite directions, without stationary

discs "; and that " The Topler machine, invented at the

same time, was a combination of two pairs of discs ";

two movable and two stationary.


He then says :

—

" Topler has recently rejected his system and adopted mine,

because it is simpler, and, at the same time, more effective. The

application of the pointed combs and the non-covered movable

discs is also my invention, since the Topler machine had only the

tin-foil coverings and sliding springs. (Schleifende Federn.)

" I had been accustomed to the same, indeed, already : although

not with independent acting, influence machines, but rejected

them on account of the smaller spark-length.

" Topler has also lately adopted my principle of the pointed

combs, and the non-covered discs : but so far modified, that besides

the pointed combs and non-covered discs, he yet allows to act. at

the same time, small pieces of tin-foil (or pieces of metal), and the

sliding springs. This has the advantage that the machine excites

itself, and is less sensitive to moisture: but also the great disad-

vantage, that the sparks become shorter, and a constant reversal

of current follows. Besides, a certain mechanic. Voss. also claims

this machine, so modified, as his merit; but unquestionably Topler

was the first who showed that influence machines, with metallic

covering and sliding springs, excite themselves.

"The entire form of the machine, its symmetrical construction,

the one-sided support of the axis, the application of a sheath

running upon a pin fastened on one side, the application of the

so-called rotary diametrical (double) pointed combs, the applica-

tion of the so-called condensers (small Leyden jars) for increase of

spark-length, is all mine, as published in the year I860, by Professor

Poggendorif (Poggendorffs Amuden, vol. 136, page 171).

" Yours truly. Dr. W. IIOLTZ."

The " sliding springs " mentioned above, doubtless

refers to a style of construction in which the springs

glide continuously over the surface of the glass ;essen-

tially different, and differing in its effect, from that of

the brushes, which touch only the raised centers of the

carriers, and are wholly insulated from the glass;

giving alternate contact and insulation, making induc-

tion much more effective. The latter construction is

attributed to Voss.

CHAPTER IX.

Experiments with the Topler Machine.

In experiments with the friction al machine, such as

the charging of Leyden jars, and .the ringing of bells,

as already described, induction is produced by connect-

ing one part of the apparatus with the earth, and

another part with the prime conductor. But in the

Holtz and Topler, the charge is accumulated in the

Leyden jars instead of on a prime conductor; and any

change of potential in one jar must be compensated by

a corresponding inductive change in the opposite jar.

Hence to obtain the full inductive effect, connection

must be made with the opposite jars.

For convenience in making this connection, holes are

drilled in the knobs surmounting the jars, and the

charge is conveyed by insulated conducting cords, hav-

ing brass tips which fit these holes.

Thus, by connecting the inner and outer coatings of

a Leyden jar or battery with the opposite jars in this

way, a full charge can be given very rapidly.

In a similar manner, image plates, bell chimes, and

other apparatus, mounted on separate stands, can be

connected and used.

Electric Chtme for Topler Machine.—Fig. 41

represents a chime designed by the author, which is

mounted on the machine itself. It consists of twobrass arms A and jB, insulated by an ebonite connector


C ; the tips of the arms being fitted to the holes in the

knobs of the jars.

A bell is suspended from each arm by a brass rod;

and a brass ball suspended by a silk cord from the

ebonite connector hangs between them.

As each bell is at the same potential as the jar with

which it is connected, the ball is alternately attracted

and repelled, causing the bells to ring.

Instruments of this kind have no practical use, except

to illustrate the principles of the science.

c Apparent Time of

the Electric Dis-

charge an OpticalIllusion.— The car-

riers on the revolving

8 plate of a Topler afford

special facilities for

this experiment. Theyare usually six discs,

arranged in a circle,

and present the ap-

pearance of a contin-Fig. 41-Chime for Topler Machine. ^^ Wlg]lt rfng ^^

the machine is operated in the light ; but when oper-

ated in the dark, they are seen only when the spark

renders them visible ; and, instead of the bright ring,

each appears by itself, apparently motionless, and as

perfect in form as if really so, just as if the movement

of the plate were momentarily arrested during the

passage of the spark.

This apparent time of the spark may be estimated

at i second; but if the carriers were really visible

during that time, the ring-like appearance would be

i r

EXPERIMENTS WITH THE TOPLEE MACHINE. 127

unavoidable, as will appear from the following calcu-

lation.

Suppose the revolving plate to have an average speed

of 4| revolutions per second, it is evident that each

carrier would make a complete revolution in less than

i second ; consequently if that were the actual duration

of the spark, each would be continuously visible round

the entire circle, and hence even a single carrier would

produce the bright ring. But it is only necessary to

this result that each should be visible until it takes the

place of its predecessor—that is during its passage of |

of the circle, which reduces the time to -is of a second.

* But if they were visible even half that time, 5V of a

second, and each were li inches in diameter, and their

distance, from center to center, 6 inches, we would

have 6 ellipses, each having a length equal to twice its

breadth.

From tlys it is evident that the smallest conceivable

duration of spark must produce an ellipse : but as each

presents the appearance of a circle, with no tendency

to elliptical form, the conclusion is inevitable that the

apparent duration of the spark is an optical illusion,

and that its time is so nearly zero, that it cannot be

estimated.

We must conclude, then, that at the instant of dis-

charge the image of the carrier is photographed on the

retina of the eye, and at the next instant darkness

supervenes: but the sensation on the retina has a mo-mentary duration, during which the carrier appears

stationary, while in reality it may have passed entirely

round the circle.

It is important to notice, in this connection, that the

appearance and disappearance of the carriers depend


on the rapidity of the discharge ; and when the spark

is made so short and rapid as to be apparently contin-

uous, the carriers appear and disappear with each snap,

like a succession of views in a rapidly moving panora-

ma, proving that the apparently continuous spark is a,

succession of sparks so rapid as to give the impression

of continuity.

As a flash of lightning is only the same thing on a

grander scale in nature's own laboratory, we must con-

clude that the passage of electricity from cloud to cloud,

a distance often of many miles, is so rapid as to defy

human calculation. We notice this in chain lightning,

when the flash, sometimes three to five miles long, is

seen throughout its entire length at the same instant,

as if suddenly photographed on the cloud.

Transmission of Power by Static Electricity.

—Two machines are necessary for this experiment

—

one called the primary, and the other secondary. The

secondary should be a very light running machine ;

hence it is better to make it smaller than the primary,

and the driving wheel and switch m^j be dispensed with.

Let the machines be placed near each other, in the

same relative position, the secondary in front; and

connected together by conducting cords or wires, joining

similar pairs of Leyden jars : and let the sliding elec-

trodes be separated beyond sparking distance. Nowlet the primary machine be put in operation, and the

movable plate of the secondary will rotate in a direction

opposite to that of the primary. If the electric energy

should not be sufficient to overcome the friction and

inertia, in starting, the plate of the secondary may be

put in rotation by hand, and its motion will then be

sustained by the electric action.

EXPERIMENTS WITH THE TOPLER MACHINE. 129

The explanation is as follows. When a Topler ma-

chine is in operation, there is a strong attraction be-

tween the plates, the result of induction from the

opposite electric states of the parts in proximity. This

attraction which constantly increases up to the instant

of discharge, acts as a resisting force which must be

overcome by the force used to rotate the plate. Now,

when the two machines are connected, this electric

force is transmitted to the secondary, where, having no

mechanical force to oppose it, as in the primary,

it causes the rotation of the plate in the opposite

direction.

Thus the mechanical force in the primary is trans-

muted into electric force, passes over to the secondary

and reproduces mechanical force ; the force applied to

the primary being expended in the secondary.

The apparatus thus becomes a scientific bank, with

its receiving and paying tellers. But nature is a

shrewd banker, and always exacts full discount; hence

the mechanical energy, paid in to the primary, is dis-

counted by friction, leakage, and heat; so that the

remaining energy may not be sufficient to start the

plate of the secondary into rotation without an ad-

ditional payment.

The sliding electrodes in the secondary machine maybe adjusted to produce the electric discharge with spark

and snap, instead of the mechanical rotation of the

plate ; thus illustrating the transmutation of force, at

will, from mechanical to electric, and from electric

either back again to mechanical, or to the heat, light,

and sound of the electric discharge.

Source of Electric Supply of the Topler Ma-chine.—The earth, the machine itself, and the air are


the only sources from which an electric machine can

derive electricity.

With the common friction machine a connection

with the earth is indispensable, and only a very limited

charge can be obtained without it ; the transfer being

either from the earth to the machine, or from the ma-

chine to the earth, as explained on page 99. Hence it

is often compared to a pump, drawing electricity from

the earth through a chain. Kemove the chain and the

supply ceases. ,x /

But with the JTopler a similar earth connection

diminishes the 6harge ; showing a loss instead of an

increase of charge. Indeed, perfect insulation of the

generating parts is an essential feature of the machine.

To demonstrate this more perfectly, let the machine

be put in operation on an insulated platform, when it

will be found that there is not the slightest perceptible

diminution of electric energy. It is evident, then, that

the earth is not its source of supply.

A certain amount is, no doubt, obtained from the

material of the machine itself; but this source would

soon be exhausted by such experiments as . the charg-

ing of a large Leyclen battery; whereas such a battery

may be charged without diminishing the energy of the

machine.

The air, then, is the only remaining source, and the

large amount of ozone generated by this machine is

conclusive evidence of its electro-chemical action on

the air, and strong, presumptive evidence that the air,

thus acted upon, has furnished the electricity whose

action has changed the oxygen to ozone.

This would imply that ozone is the result of depriv-

ing air of a portion of its electricity; whereas if the


electricity were derived from the earth, we must infer

that its generation precedes the generation of ozone,

instead of being coincident with it. But the insulation

proves that the earth does not supply the electricity;

so that the weight of evidence is in favor of ozone being

the direct result of electric generation, rather than a

result of subsequent electric action. And, if such is

the case, it is strong proof that the air is the chief

source of electric supply.

The generation of ozone by atmospheric electricity

during thunder storms is a well-known fact; and clouds,

-floating miles above the earth, must obtain their elec-

tricity either from their own vapor, or the air, or both.

Such clouds, at different electric potentials, insulated

from the earth, acting inductively on each other, and

finally producing a discharge, fulfill the same conditions

as exist in the Topler machine ; and the generation of

ozone is doubtless due to the same cause in both. Andsince the vapor of the cloud corresponds to the material

of the machine, and it has been shown that the electric

supply of the machine from its own material must be

very limited; and since the machine operates mosteffectively in a dry atmosphere, and hence does notderive its electricity from vapor ; we may infer that the

electric action is the same in both cases, and that the

air is the chief source of electric supply.

It is evident from the movement of particles of dustand other light bodies towards the machine, that the

air in which these atoms float must have a similar

movement ; that currents of air are constantly flowing

to the machine and that this air, after being changedto the same electric potential, is repelled, and air at a

different potential flows in to take its jolace ; a move-


ment similar to that which takes place in the hot and

cold currents round a heated stove.

But the initial charge is undoubtedly from the ma-

terial of the machine itself, and results from the friction

of the brushes on the carriers ; after which follows the

increase by induction and the action on the air.

Electricity Generated by the Friction of

Metals.—The old division of all substances into elec-

trics and non-electrics was the exponent of the idea

then prevalent, that only certain substances, as glass,

sealing-wax, and other non-conductors, comprised in a

very brief list, were capable of electric excitation.

While this view is no longer maintained, yet, since in

nearlv all experiments illustrating the elements of static

electricity, glass, sealing-wax, ebonite, silk, wool, fur,

and other non-conductors, are almost exclusively em-

ployed as generators, we are apt to lose sight of the

fact that metals and other conductors are capable of

generating electricity by their mutual friction. Andyet this is one of the most important principles of static

electricity. It is that which liberates our ideas of elec-

tricity from the narrow bounds to which they were

once confined, proving that it is not a special property

of certain substances, but a universal property of mat-

ter, one form of that energy which pervades and con-

trols the universe.

This point has been already illustrated, but the

Topler machine affords special facilities for illustrating

it more fully. In it the initial charge is produced hy

the friction of -metal brushes on metal carriers. True,

both carriers and brushes are attached to glass, and the

glass subsequently acts by induction as a generator;

but the friction is confined to the carriers and brushes


alone; and, so far as the electricity is obtained from

this source, the glass acts only as an insulator to pre-

vent the escape of the electricity generated by the

friction, from which the initial charge is derived.

It is not even necessary that the metals should be

different. The machines here described are constructed

with brass carriers, and brushes of brass wire ; and,

though the carriers are nickel-plated, so that the friction

is that of brass brushes on a nickel surface, yet carriers

left unplated give equally as good results.

The Spark; Its Direction, Subdivision, andColor.—The spark from a Topler machine presents

phenomena which demand careful investigation.

As already shown, the apparent time of the discharge

is an optical illusion, time being practically annihilated;

so that it is impossible, from observation, to tell in what

direction the discharge takes place. A brilliant streak

of white light, extending from one electrode to the other,

suddenly appears and disappears, leaving us in igno-

rance as to the direction in which it moves. But the

following experiment affords better opportunity for

observation.

As already shown, the electric connection between the

inside coatings of the Leyden jars may be interrupted

by separating the sliding electrodes, and that between

their outside coatings by opening the switch. Put the

machine in operation in a darkened room at night, with

the switch open, and the sliding electrodes separated

three or four inches. From the electrode P, Fig. 42,

a brush of violet-colored light, diverging from a small,

circular space, extends about i of an inch towards the

opposite electrode, accompanied by a hissing sound.

The opposite electrode, it, remains comparatively qui-


escent at first, showing only a glow of light ; but, as

the electricity accumulates, there is a suclcleii outburst

from it, accompanied by phenomena of the most inter-

esting and varied character.

A brush of light, of a faint white, or violet color,

darts across the intervening space, diverging towards

the center, and converging as it meets the brush from

the opposite electrode ; forming an elliptical figure, two

or three inches in diameter, extending from one elec-

trode to the other. Through the center of this brush

shoot out long tongues of red and violet light, curving

and branching in a variety of fantastic forms. Some-

times five or six of these appear at once, like fiery

serpents, hissing, spitting, and darting out their red

forked tongues. Sometimes the appearance is that of

a miniature tree, its main trunk branching off at various

angles and curves. Then, again, the brush disappears,

and we have a single, straight, violet colored stem,

about f of an inch long, which divides into a great

number of bright rays, radiating in straight lines from

the end of the stem, and forming a globe of white light,

about three inches in diameter: the whole resembling

a little bush of remarkably regular appearance, in

marked contrast with the curved and contorted phe-

nomena just described.

Between this, and the short brush on the opposite

electrode, a dark space intervenes, into which the rays

pass and intermingle ; the brush from the electrode Rbeing largely in excess of the other, and showing far

greater energy ; but more fitful, coming at first in jets,

with a spitting sound, while the other is more constant,

with a steady, hissing sound.

As explained on page 118, electric movement is from


the comb L downwards into the jar D, then along the

switch, when closed, and its connections to C\ and up-

ward out of to the plate and carriers ; and, in part,

alone the electrode P towards It, which connects with

the inside of D: the glass of the jars, and the air space

between P and B forming barriers at which electricity

Fig. 42—Experiments with the Toplei Electric Machine.

accumulates on the side towards which the movement

takes place, induction producing a corresponding neg-

ative on the opposite side.

D has the higher potential, as already shown, but its

inside charge is bound by an equal negative on its

outer coating; while electricity is repelled from the


inside of (7. Hence, when the switch is open, we have

the difference in the brush discharge already described.

But as the higher charge of D continues to accumu-

late on its inside coating, the tension increases on the

electrode i2, till the electricity finally bursts through

the resisting air from B to P ; producing the spark and

snap when the switch is closed, as already explained.

The effect of opening the switch is to substitute for

this metal conductor, which has comparatively no

resistance, a portion of the base, which is of kiln-

dried wood, and offers high resistance. This retards

the current, producing the difference of phenomena

between the bright, instantaneous spark of white light,

with its sharp report, and the slow moving brushes of

violet light, with their hissing, spitting sounds; and

from this slow movement we are able to determine the

direction of the discharge, as already shown.

The cause of the subdivision of the spark when the

switch is open next claims attention. It has been

shown that the discharge between the inside coatings

through the electrodes P and li, and the intervening

air space, is dependent on the counter discharge

between the outside coatings, through the switch, whenclosed, or, through the kiln-dried wood of the base,

when the switch is open. This discharge may be seen

by opening the switch, half an inch or more, so that

the resistance of the air is less than that of the wood.

We then have a bright spark below, simultaneous with

the spark above. But when the switch is opened so

that the resistance of the air is greater than that of the

wood, the discharge below takes place silently through

the wood, and we have above, the subdivided, colored

discharge already described.


With the switch closed, reducing the resistance below

to zero, the discharge through the air is instantaneous

;

and there is seldom any subdivision, except that a long

spark from a heavy charge sometimes divides into two,

slightly separated during a part of their course. But,

with the switch open, the high resistance retards the

lower discharge, which is compelled to force its way

slowly through the kiln-dried wood; making the

change of potential between the outside coatings slow

and gradual, and producing a similar effect on the

inside coatings. Now, as the spark is caused by the

electricity forcing its way through the air, whose elec-

trified molecules are at the same potential near each

electrode, and hence self-repellent, while the surround-

ing air is at a lower potential and attractive, these

forces, acting in part at right angles to the original

impulse, during the comparatively slow progress of

the discharge, produce the brushes of diverging rays

already described. Various influences, such as currents

of air, particles of dust, and the induction of electricity

generated on adjacent parts of the machine, curve and

contort the spark, producing the peculiar phenomenaalready described in connection with the brushes, and

also affecting the long bright sparks in a similar manner..

We next notice the color of the spark. Light is a

mode of motion, and its color is influenced by the

intensity of the motion. A bar of iron, drawn from the

furnace, ready for rolling or welding, is said to be at a

white heat; as it cools it changes to a red heat. Herethe color of the light depends on heat, which is also a

mode of motion ; and as the intensity of the heat mo-

tion decreases, the light changes from white to red of

various shades, till the bar resumes its original color.


The brilliancy of the arc in the electric lamp is due

to the intensity of the motion, while the softer light of

the incandescent lamp results from a motion less intense.

When an electric lamp is being lighted or extinguished,

the change of color from white to the various shades of

red is evidently dependent on decrease of motion.

Must we not 'conclude then that the white light of the

electric spark, when the switch is closed, is due to

intensity of motion, and the colored light with the open

switch, to decrease of intensity, as in the iron bar or

the carbon of the electric lamp ? Or, if light and heat

are modes of motion, is not the evidence equally strong

that electricity is a mode of motion? Or may we not

go still farther, and say that light, heat, and electricity

are only different manifestations of that energj' which

is a universal property of all matter, of which the ex-

periments here given are an additional proof? For in

the electric spark, we have light, heat, and electricity

combined.

Having stated that D is usually the jar of higher

potential, it should be explained, that there is fre-

quently a temporary reversal of potential; and, whenthis occurs, all the phenomena here described are

reversed also. The cause of this reversal will be ex-

plained in connection with the following experiment.

Direct axd Reversed Rotation.—A Topler ma-

chine can be charged only by revolving the smaller

plate in a given direction; which, in the machine

represented, is shown by the arrow.

The reason is this: In order to store up electricity

in the Leyden jars, each carrier must pass from an

insulated brush, where it is charged, directly to a comb

connecting with a Leyden jar, before it passes to an


uninsulated brush, where it is discharged. Thus the

carrier TT, charged by the friction of the insulated

brush E, must pass to the comb Z, connecting with the

jar D, and give up its principal charge, before passing

to the uninsulated brush and comb JT, where its resid-

ual is discharged through the brass rod H V, which

puts it in electric connection with the carrier Z, of

opposite potential. Reverse the rotation, and the

carrier IF, starting from E, would give up its princi-

pal charge to the uninsulated brush and comb at V,

before reaching the comb K, connecting .with the

Leyden jar (7, where only the residual would remain.

It must also be noticed that the charge is greatly

increased, both on the carrier and adjacent portion of

the plate, by passing the inductor I7

, attached to the

stationary plate B ; whereas, when the rotation is re-

versed, the carrier leaves the inductor and passes the

space between T and X, where the induction is almost

zero. Thns it is evident that no storage of electricity

in the Leyden jars, and hence no permanent charge can

be obtained from a reversed rotation.

Higher Potential of Jar, D.—It has been shown,

that from the higher position, and hence better insula-

tion of the brush J?, and upper half of the revolving

plate J., as compared with the lower position, and con-

sequent inferior insulation of the brush F, and lower

.half of A, the potential of the jar D, receiving its

charge from the former, must be higher, as a rule, than

that of (?, which receives its charge from the latter.

Repeated experiments, made by the author with a

number of different machines of this kind, fully confirm

this view. The higher potential is shown by the fre-

quent partial discharges between the inside and out-

140 ELEMENTS. OF STATIC ELECTRICITY.

side coatings of this jar ; and, in case of fracture, which

sometimes occurs from an overcharge, it is always this

jar which is broken : and the fracture always occurs on

the side nearest the opposite jar, showing that the

charge is attracted to that side, and electricity repelled

from the outside coating, creating a sufficient difference

of potential between the two coatings to overcome the

resistance of the glass and perforate it.

Reversal of Potential.—It has been already

stated that there is frequently a temporary reversal of

potential. Such a reversal can be produced, if desired,

by joining the electrodes P and i?, and reversing the

rotation of the plate till the machine is fully discharged;

then separating P and 11 while the reversed rotation is

continued, and then resuming the direct rotation, whena complete reversal of potential will be found to have

occurred, which will continue till again reversed by a

similar experiment, or till the machine has had a period

of rest. The explanation is as follows:—When P and 11 are separated, and the rotation re-

versed, the same causes which before operated to raise

the potential of D above that of (7, now operate to raise

the potential of C above that of i), but in a very lim-

ited degree. For, as already shown, any carrier, as IF,

charged by the brush E, would now give up its princi-

pal charge to the brush and comb at V; but the resid-

ual, slightly increased by the inductor X, would be

given up, through the comb iTto the jar O; while the

opposite carrier Z, would give up its principal charge

at H, and carry its residual to the comb L, and the jar

D, after a slight increase by the inductor T. But the

difference of insulation between the upper and lower

parts affect these residual discharges in the same man-


ner as the principal discharges, and hence operate to

make the potential of (7, receiving its charge from

above, higher than that of Z>, receiving its charge from

below. This residual is not sufficient of itself to bring

the machine into action, but it creates a slight differ-

ence in favor of (7, sufficient to sustain a reversal of

potential when the direct rotation is resumed.

The Faradic Current.—The faradic current con-

sists of a series of electric impulses following each other

with great rapidity. It is obtained from the battery

and coil by a spring vibrator, which opens and closes

the circuit; and from the magneto-electric machine by

a revolving electro-magnet and commutator.

Both these instruments have, for many years, been

extensively used in medical practice ; but the use of a

static machine for this purpose is quite recent, and the

switch, on the machine here represented, affords special

facilities for producing and utilizing this current. In

Fig. 42 are shown two sockets, on the front edge of

the base, connecting with the terminals of the switch,

into which are inserted the tips of conducting cords, to

the outer extremities of which may be attached metal

handles, as shown, or other electrodes suitable for the

use of this current, for medical or scientific purposes.

As already explained, when the machine is in oper-

ation there is a constant movement of electricity

through the switch and its connections, from I) to (7,

while the charge is accumulating; and the counter dis-

charge through them, from C to 7), is simultaneous with

the discharge above, from R to P. When the switch is

open and the cords attached, as shown, this discharge

must either force its way through the kiln-dried wood,or pass out through the cords and any object connected


with their outer terminals, according to the degree of

resistance offered by each path respectively. If a per-

son, or a number of persons with hands joined, grasp

the handles, the resistance will be less than through the

wood, and they will feel the effects of the discharge.

This discharge is regulated by the distance to which Rand P are separated. With a separation of TV of an

inch, on a large machine, the discharge is so rapid that the

distinction between the impulses can scarcely be per-

ceived; producing a faradic current smoother than can

be obtained from the best batteries, while a separation

of 2 an inch produces effects which the strongest nerves

cannot endure.

This current, in its milder form, cannot be distin-

guished from that obtained from the battery, or mag-

neto-electric machine : but, in its more powerful effects,

it is more impulsive; coming in jets, with cumulative

force, like the rapid blows of a planishing hammer. In

the battery current, the stronger effects show increased

intensity, and a greater tendency to muscular contrac-

tion ; while increase of strength in this current is

due to the slower impulses giving more time for the

accumulation of electric energy.

The Electric Bath and Electric Wind.—Charg-

ing a person on an insulated stool is one of the most

common experiments in static electricity, but it has

only recently come into use in medical practice; and,

instead of the stool, an insulated platform, on which

one or more persons can be comfortabty seated, has

been substituted; the treatment being known as the

"Electric Bath."

When the patient is seated, as above, the electrodes

P and i£, drawn out beyond sparking distance, and the


switch closed, a connection is made between the pa-

tient and the machine by a conducting cord; one end

being attached to the ball surmounting one of the

Leyden jars, and the other end to the chair. A similar

connection is made between the opposite jar and the

floor near the platform, to create a certain degree of

induction, and so facilitate the process of charging,

which is now done by putting the machine in oper-

ation. Very little sensation is experienced from this

charge, but its effect in certain nervous diseases, which

cannot be treated with the battery, such as St. Vitus

dance, is said by medical men to be very soothing. In

other cases, sparks are drawn from the patient with the

hand or a suitable electrode, as a ball, roller, or sponge,

attached to the cord from the opposite jar, and held by

an insulating handle.

The electric wind is given by a point electrode,

attached as above, either with or without the insulated

platform. A gentle current of electrified air from the

point fans the patient, producing a delightfully sooth-

ing sensation.

Electric treatment of this kind can be given only bystatic electricity, and its value must be determined bythe medical profession, among whom it is coming into

favor; being used and recommended by physicians of

eminence.

Gas Lighting.—Lighting the gas in churches andpublic halls by electricity is commonly done by a bat-

tery and coil, but the Topler machine can also be usedfor this purpose. With either method there must bewires connecting the generator with the chandeliers,

wires connecting the chandeliers together, and also the

separate burners; all arranged in one circuit and prop-


erly insulated. At each burner there is a break in the

circuit, so arranged that a short spark will pass through

the gas; the ends of the wire being attached to an

insulator fitted to the burner.

With the battery there is a ground wire, and con-

nection with the gas pipe to complete the circuit; but,

with the machine, the circuit is made by two separate

wires, connecting the chandeliers with the balls sur-

mounting the Leyden jars. On account of the greater

intensity of static electricity, these wires must be thor-

oughly insulated with thick rubber tubing, wherever

they are liable to come in contact with the walls or gas

fixtures. With these arrangements properly made, it

is only necessary to close the switch, separate P and Rto the full extent, turn on the gas, and put the machine

in operation. The resistance of the air between P and

i?, being greater than the resistance of the wires and

the short breaks between their terminals at the burners,

the sparks take place at the burners, and the gas is lit.

As to the expense, convenience, and efficiency of this

system, as compared with the battery system, only gen-

eral statements can as yet be made. The first cost

would probably be about the same; after which there

would be no further expense with the machine, which,

with proper care, should remain in good working order,

for this purpose, for an indefinite term of years; while

the battery requires frequent renewal of the fluid, and

occasional renewal of the zinc, besides cleansing and

amalgamating.

As to efficiency, the greater intensity of the spark

from the machine will be evident, when we consider

that a machine of very moderate size will easily pro-

duce sparks three to five inches in length, while a very


large battery and coil would be required to produce the

same result. But this should be taken merely as an

indication of comparative intensity ; as, practically, only

very short sparks are required : so that a battery and

coil of medium size is generally sufficient.

A damp atmosphere does not affect the battery,

while it lessens the energy of the machine; and, in

unskillful hands, may interfere with its practical

efficiency. But, with either system, the person in

charge should have a thorough knowledge of its care

and management: in which case the machine can

always be kept in practical working order.

CHAPTER X.

Electric Transmission in Vacua.

Electric Transmission in Low Vacua.— Let a

glass tube, about thirty inches long, be provided

with brass caps at each end, fitting air tight; from

each of which a pointed brass rod projects inwards.

And let a stop-cock be attached to one of the caps, by

which the tube can be connected with an air pump, as

shown in Fig. 43.

Let the tube be insulated, and the caps connected by

conducting cords with the balls surmounting the Ley-

den jars of the Topler machine; the sliding electrodes

being separated to their full extent. When filled with

air, at the ordinary atmospheric density, it will be found

impossible to pass an electric charge through a tube of

this length: but let it be connected with an air pump,

and the air well exhausted, and a charge will easily pass

through. This proves that air at the ordinary density

lias a much higher electric resistance than rarefied air.

But if a high degree of vacuum is produced, it will be

found much more difficult to pass the charge through

:

which indicates that a medium, consisting of some

material substance, is essential to electric existence

and movement; and that if it were possible to produce

an absolute vacuum, electricity could not pass through.

If the above experiment be performed in a dark room,

flashes of red and violet colored light will be seen to

ELECTRIC TRANSMISSION IN VACUA. 147

accompany the discharge, strongly resembling the cor-

uscations of the aurora polaris. Hence tubes, used for

this purpose, have been called aurora tubes.

Geissler Tubes —Improved tubes of this

kind, called from their inventor Geissler tubes,

are constructed with fine platinum wire sealed

into their extremities; the points projecting

inwards, and loops formed outside for the

attachment of conducting cords or wires. The

glass is bent into a variety of graceful curves

and folds: small tubes, bent in this manner,

being inclosed, for protection, in large straight

ones; and thus long, frail tubes are reduced to

compact, convenient forms, in which they can

be safely handled, as shown in Fig. 44.

The air is exhausted from them by a mercury

pump, after which they are hermetically sealed.

The expansion of the fine platinum wires being

very slight and nearly the same as that of the

glass, is not sufficient to cause fracture, hence

the vacuum produced in well-constructed tubes

remains permanent for years.

Beautiful fluorescent effects are obtained byconstructing such tubes of uranium glass. Sim-

ilar effects are also obtained by introducing into

them.various solids and gases; as sulphate of

quinine, fluoride of boron, fluoride of silicon, (§§

iodine, hydrogen, and nitrogen ; which give

certain characteristic colors, when subjected to

electric action. £jg. 43—Vacuum

The effect of the discharge is greatly increased Tube -

if a break be made in the connection between one endof the tube and the machine, so as to introduce a short


Fig. 44—Geissler Tubes.


spark into the circuit. The electricity then accumulates

and a rapid succession of brilliant discharges is the result.

The same effect can be produced by opening the

switch, connecting the tube with its terminals, andslightly separating the sliding electrodes P and P.

When the spark between P and R is apparently

continuous, the pulsations in the induced discharge

through the tube are distinctly visible in the alterna-

tions of light and shade ; proving that the discharge

consists of a series of distinct impulses.

Fig. 45—Rotary Movement in High Vacua.

Electric Transmission in High Vacua.—The re-

sidual air in the ordinary Geissler tube is about -nfiftfflW

of an atmosphere, but Crookes has produced tubes in

which the residual is less than T oo<W<7o of an atmos-phere ; and the electric discharge in such tubes presents

certain peculiarities not observed in ordinary vacua.

Electric action on substances inclosed in such tubes,

and on the glass itself, is increased in the ratio of the in-

creased vacuum; since those substances receive the force

of energy which, in lower vacua, is expended on the air.

This increased action is shown by an increase in the

light and heat developed in them, and in the attractive


force exerted on them, as shown when they are free to

move.

Fig. 45 shows such a tube, having a glass railway on

which is placed a roller with mica vanes, and the

electrodes so placed that the upper vanes are in line

between them. When an electric current passes

through the tube, these vanes, being at zero potential,

are attracted by the higher potential of the positive

electrode, producing a rotary movement of the roller

from negative to positive ; the force being sufficient to

move it up an incline.

Fig. 46—Rotary Movement Reversed.

Fig. 46 shows a tube in which a wheel with mica

vanes is so mounted that its center is in line between

the positive electrode, and the center of the negative.

The negative electrode a b is cup-shaped, and its con-

cave surface turned towards the positive : so that the

lines of force may be brought to a focus, and concen-

trated on the vanes. And between its center and the

wheel is placed the mica screen c d.

A magnet, #, is suspended above the tube, between


the screen and negative electrode, in such a manner that

it can be turned so as to reverse the position of its poles.

By this means the electric current may be attracted

or repelled, so as to pass over or under the screen.

When it passes over the screen, the upper vanes are

attracted towards ihe positive electrode, producing

rotation of the wheel in accord-

ance with such movement : but

when the position of the magnet

is reversed,the current is repelled

and passes under the screen, and

the lower vanes are attracted,

reversing the rotation.

The glass in these tubes is

usually of very low insulating

power, much lower than that

of air at the ordinary density.

Hence the electric resistance in

high vacua, being much greater

than in the glass, electric move-

ment takes place through the

vacua and glass respectively, in

the inverse ratio of the resist-

ance of each.

Fig. 47 represents a tube in

which the negative electrode

consists of a half cylinder of

aluminium, supported, near the center of the tube, on a

small glass tube, b; through which a copper wire ex-

tends, connecting the aluminium with the platinum

terminal below.

The ends of both electrodes come near the walls of

the tube; and when the electric charge passes, its prin-

Fig. 47—Glass Illuminated


cipal effect is produced on the glass, which gives a brill-

iant green light ; the illuminated surface terminating in

points near the extremities of the negative electrode.

The influence of induction on the walls of the tube;

as well as the conductivity of the glass, is illustrated in

Fig. 48 ; which represents a pear-shaped tube, having

for its positive electrode an aluminium cross, 6, placed

near its broad end; the negative electrode a being

cup-shaped as in Fig. 46. This cross is hinged at bot-

tom to the platinum terminal; so that, by a movementof the tube, it can easily be brought to a horizontal or

a vertical position.

Fig. 48—Inductive Action of Metal Screen.

When the charge is passed through the tube, the

cross, when vertical, as shown in the cut, exerts a strong

inductive influence on the broad end of the tube, to the

left ; over a space inclosed by lines extending over its

edges, from the negative electrode : repelling electricity

from this space, and screening it from the action of the

negative electrode, which attracts electricity from the

other parts of the tube, and from the surrounding air.


Hence electric action within this space is neutralized

;

producing the dark shadow c d shown on the broad

end ; while the rest of the tube is illuminated.

• When the screen is thrown down a luminous cross

takes the place of the dark shadow: but this higher

illumination soon fades, since electric action on this

space is now the same as on the rest of the tube.

If the tube be used again, after a period of rest, the

shadow can be reproduced; but is never so strong as at

first. This proves that the glass has been subjected to

an electric strain, which has permanently lessened its

insulating power.

The illumination of the glass is due to its resistance

;

just as the bright spark is due to the resistance of air

at the ordinary density, and the faint glow, to the

reduced resistance in vacuum. Hence, when electric

action begins, after the screen is thrown down, the

resistance being greater on the spot which was pro-

tected by the screen, we have the bright cross where

the dark one was : but when the electric strain has so

affected the relations of the molecules to each other, as

to lessen the resistance, this first bright glow ceases, and

the illumination is the same as in the rest of the tube.

This action on the glass, as shown in Figs. 47 and 48,

is accompanied with heat as well as light ; the tube

shown in Fig. 47 becoming intensely hot, at those

points where the greatest electric energy is concen-

trated.

Fig. 49 represents a tube constructed to show this

heating effect in a very striking manner. Its upper

part is enlarged into a globular form : and, at the bot-

tom, is the concave negative electrode, of aluminium,

already described ; which is so placed that it brings


the lines of force to a focus on a piece of iridio-platinum,

5, placed in the center of the globe. This, being a

metal of high resistance, becomes white hot under the

electric action ;glowing with intense brilliancy, and

finally melting.

The walls of the globe, being remote from the line

between the electrodes,

which is comparatively

short, the glass is less af-

fected than in the long

narrow tubes: so that elec-

tric action is chiefly con-

centrated on the object at

the center.

Crookes attributes all

these phenomena to the

impact of the residual air

molecules, which he desig-

nates as "radiant matter";

and claims that the mole-

cules move independently

of each other, and are

driven with such force

against the glass and other

objects, as to produce the

various phenomena de-

scribed.

Gordon considers this theory reasonable, and elabo-

rates it at considerable length : but it is not generally

accepted ; and it is believed that the explanations here

given will be found more in accordance with well estab-

lished electric principles.

Fig. 49—Heat Produced in HighVacua.

CHAPTER XI.

Electrometers.

Progress in every department of science is largely

dependent on exact measurement, since it is only by

this means that we get an accurate knowledge of

relative values. The thermometer enables us to in-

vestigate the laws of heat; the barometer gives us a

knowledge of atmospheric pressure, and the various

matters relating to it. And in chemistry and astron-

omy almost every step depends on such measurement.

Even our ordinary business transactions, and the value

of our currency, are regulated by the common scales,

by which we measure the force of gravity.

Electric science is no exception to this rule. Werequire to know, accurately, relative differences of

potential; the conductivity and resistance of various

substances ; the force of electric attraction and repul-

sion, the comparative energy of the various instruments

used for generating and accumulating electricity ; and

other matters of similar importance.

But electric measurement presents peculiar difficul-

ties not met with in the measurement of other forms

of energy. In the measurement of gravity, we deal

with a force easily controlled, the direction of whose

movement is always known, and which, on the various

parts of the earth's surface, is subject to but slight

variation.


In heat we have a force, susceptible of easy control

;

its movement slow, and its direction easily ascertained.

But electricity moves with the rapidity of thought;

its direction is difficult to ascertain ; and it defies our

utmost efforts at absolute control; so that the results

of measurement, by our best constructed instruments,

fall short of perfect accuracy.

In static electricity less progress has been made in

measurement than in other forms of electric energy,

whose practical applications are more numerous.

The electroscope, sometimes

classed with electrometers, in-

dicates the presence of an elec-

tric charge, but cannot be said

to measure it, except as such

indication may show an in-

crease or diminution of a light

charge. Lane's unit jar maybe considered an electrometer,

and the methods of measure-

ment by it, and by sparks

from the Holtz and Topler

machines, belong to the same

class : but both methods are

very inaccurate, and can be

used only in special cases.

Coulomb's Torsion Balance.— To Coulomb is

due the credit of the first efforts at accuracy in elec-

tric science ; and the torsion balance, which is still

extensively used, was his invention and may properly

be regarded as the first electrometer.

It is represented by Fig. 50; and consists of a glass

cylinder A A, to the top of which is attached, at the

Fig. 50- -Coulomb's TorsionBalance.

ELECTROMETERS, 157

center, a glass tube D D, to each end of which is fitted

a brass collar. An enlarged section of the upper end

of this tube and its attachments, representing what is

known as the torsion head, is shown separately; in

which it will be noticed, that the brass collar a has

fitted to it a cap b with a projecting rim ; on the

upper surface of which is a graduated scale, of 360

equal divisions. This cap is capable of being turned

horizontally, so as to bring the several divisions of the

scale under a pointer <?, attached to a.

In the center of b is a close fitting brass rod cZ,

with a broad head by which it can be turned, when b

is held firmly ; or the rod may be allowed to turn with

b. Attached to this rod is a fine wire, which sustains,

at its lower extremity, a horizontal shellac rod /,

carrying at one end a small gilt ball g. Opposite this

ball, on the cylinder A A, is a graduated scale a a,

having 360 divisions, to correspond to those of the

upper scale. Opposite the zero of this scale is a gilt

ball g\ of the same size as the other gilt ball, and sup-

ported by a shellac rod fn

', by which the ball can be

introduced through an opening in the top of A A.

The instrument is supported on a base, having level-

ing screws ; and the air, in the interior, kept dry with

chloride of calcium.

To use this instrument, the cap b is turned till the

zero of the upper scale is brought under the pointer c.

The rod d is then turned till the movable ball g just

touches the fixed ball g\ without torsion of the wire.

The zeros of the two scales will then be practically in

the same vertical plane.

The fixed ball <f is then taken out, electrified, and

replaced as before, in contact with the movable ball g.


Both being the same size, the charge is equally divided

between them ; and, being at the same potential, the

movable ball g is repelled to a distance indicated by

the number on the lower scale : at which point the

force of repulsion is balanced by the torsion of the wire.

The cap b is then turned in opposition to the repul-

sion, so as to bring the ball g nearer to g'; the distance

being indicated on the upper scale. The torsion of the

wire is thus increased, and repulsion again balanced by

torsion in the new position.

It is known that the force of torsion is proportional

to the angle of torsion : and since this force has to be

increased to oppose the increase of repulsion, as the

balls are brought closer, the point to be determined is

the ratio of increase of force, as compared with the

reduction of distance between the balls; which is done

by comparing the distances from zero indicated on the

upper and lower scales.

The following is one of Coulomb's experiments for

this purpose : The first distance to which the movable

ball g was repelled being 36°, it was found necessary,

in order to reduce this distance to 18°, to turn the cap

b through 126°; and to reduce the distance to 8J° re-

quired an additional rotation of the cap through 441°.

The distances 36°, 18°, and 8i° are to each other,

practically, in the ratio of 1, £, and I ; and the forces

of repulsion at these points were balanced by torsions

of 36°, of 126°+ 18°= 144°, and of 441° 4 126°+ 8J°=

575i°, respectively.

Now since 144= 4x36, and 575J (practically 576)

=16x36, it will be seen that as the distance between

the balls is divided by 2 or by 4, the force of repulsion

is multiplied by 4 or by 16 ; and thus Coulomb proved

ELECTROMETERS 159

that electric repulsion varies inversely as the square of the

distance.

Inaccuracy of the Torsion Balance.—In the

use of this instrument, as above, the arc is assumed as

the distance between the balls, while the actual dis-

tance is the chord of the arc ; but since these distances

are in the same proportion, the accuracy of the results

is not affected.

It is also assumed that the arm of the lever, by which

repulsion produces torsion, is the distance from the

center of motion to the cen-

ter of the ball g. But this

is true only when the balls

are in contact. In every

other position, this arm is

represented by a perpendic-

ular from the center, on the

chord which cuts the centers

of the two balls : and as the

ball g moves round, and the

chord increases in length,

this perpendicular decreases;

and vanishes when the chord

equals the diameter.

This is shown in Fig. 51, where b represents the first

position of the balls, when the arm equals a b : but

when g moves round to c, the line a f represents the

arm ; and when it moves to d, the short line a m rep-

resents the arm ; and at n the arm vanishes.

This may be made more plain, by considering that

the ball g is moving under the influence of two forces,

electric repulsion, and the rigidity of the shellac rod,

by which it is held at a fixed distance from the center.

Fig. 51—Arm and Angle ofRepulsion Illustrated.


When motion begins, at 5, these forces act at right

angles to each other but as the ball moves round, the

angle of repulsion constantly decreases ; being repre-

sented at e, by the angle a b c; and at cZ, by the

angle a b d; and vanishing at ti, where the two forces

are in direct opposition.

In this position the force of repulsion opposes further

movement : for, as it radiates equally in every direction

from the two balls, its force on opposite sides of n is

equal. But since, in the experiment given, the greatest

angle was 36°, which is only one-fifth of the semi-circle,

the error is not sufficient to affect the result seriously.

Another inaccuracy results from lack of rigidity in

the fine wire, which causes it to deviate slightly from a

true vertical, under the influence of repulsion ; moving

its lower extremity out of the center.

There is also a slight inaccuracy resulting from the

force of repulsion being estimated from the centers of

the balls, instead of from their nearest points.

It is also assumed that electric repulsion remains

constant during the experiment : which would not be

strictly true; since there is a continual reduction of

energy, from causes already explained, which would

produce serious error, if the experiment were of long

duration.

Since each ball becomes a center of electric radiation,

it is evident that the lines of force cut by each repre-

sent but a very small part of the entire repulsive energy.

But since the balls are of equal size and equal poten-

tial, it may be assumed that the proportion between

the energy actually measured, and the entire energy, is

the same in each ball. But an instrument embracing

all the lines of force would evidently be more reliable.

ELECTROMETERS. 161

These inaccuracies doubtless account for the slight

error observed in Coulomb's experiment, and tend to

confirm the correctness of his results by showing suf-

ficient cause for the error.

Atteacted-Disc Electeometees.—Sir W. Snow

Harris was the first to construct an electrometer on the

attracted-ciisc principle. His instrument consisted of a

scale beam, carrying at one end a pan for the weights,

balanced at the other end by an insulated metal disc, sus-

pended horizontally over a similar fixed, insulated disc.

An electric charge being given to the lower disc, the

force of attraction between it and the upper disc was

measured by weights placed in the scale pan.

The rapid loss of charge, from the edge of the elec-

trified disc, was the chief objection to this instrument.

But the principle has been adopted, and the construc-

tion improved by Sir William Thomson, whose instru-

ment, shown by Fig. 52, is described as follows:

—

Thomson's Absolute Electeometee.—This in-

strument consists of two distinct parts ; one for testing

and maintaining a certain constant auxiliary potential

V, and the other for determining, in absolute measure,

the difference between the potentials of any two given

conductors. The first of these parts embraced a Ley-

den jar, forming the case of the instrument, an idio-

static gauge, and a replenisher E.

The Levden jar is a glass cylinder, closed at top andbottom by metal plates: and coated, inside and out,

with tin-foil, in which openings are left for viewing the

internal parts; and an uncoated surface, for insulation,

left at the top and bottom, between the inner coating

and the metal plates.

The idiostatic gauge will be understood from Fig.


Fjcr. 52—Thomson's Absolute Electrometer.

ELECTROMETERS. 163

53. A small aluminium plate P is fitted to a square

hole in the metal plate G-, like a trap-door, without

touching the edges. To one side of P is attached an

arm A, of the same material, enlarged at its junction

with P, and bent, so that when the surfaces of P and

Gr are in the same plane, the arm is elevated a little

above Gr, and is parallel with it.

A platinum wire f stretched between tw^o supports,

attached to Gr, passes through the enlarged part of the

arm A, over a slight projection ; supporting P, and, by

its torsion, regulating its movements. At the outer

end of the arm is a fork

F ; and between its

prongs is a little white

enameled standard, at-

tached to Gr ; having,

on its outer face, two

black dots, close to-

gether, and in the same

vertical line. A black

hair, stretched across the fork, and viewed through the

lens ?, moves up and down in front of the dots ; and

comes exactly between them, when the surfaces of Pand Gr are in the same plane. This is called the sighted

position.

Under the plate Gr is seen, in Fig. 52, a circular

metal plate P, supported on a metal rod, attached to

the metal plate A, which is in contact with the inner

coating of the Leyden jar; so that A and Pare always

at the same potential F", as this coating. The distance

between P and Gr is so regulated, that when the poten-

tial of P is V, its attraction for the plate P overcomes

the torsion of the platinum wire, and keeps P in the

Fig. 53—The Idiostatic Gauge.


sighted position : and, in this way, the constancy of the

potential Fis tested.

This constancy of potential is maintained by the

replenisher seen at E, which is practically a small To-

pler machine ; and is shown separately in Fig. 54. Aand B are two insulated metal inductors, to which are

attached two receiver springs a and b. C and D are

two contact springs, in electric connection with each

other, but insulated from the other parts.

P and Q are two metal

carriers, attached to an eb-

onite cross-piece, through

which passes the ebonite

axis T7

, which can be ro-

tated by the milled head

E: so that the carriers

P and Q, revolving inside

the inductors A and B,

shall successively touch

the springs a, 2), 5, C.

When the replenisher

is in its place, as shown

in Fig. 52, the inductor A is put in electric connection

with the disc A ; which is supported in connection with

the inner coating of the Leyden jar : while the inductor

J9, being in contact with the cover, is in electric con-

nection with the outer coating. And since the replen-

isher operates on the principle of the Topler machine,

already described, its rotation, either direct or reversed,

will raise or lower the potential of the jar : and so keep

the potential of the plate A, and of the idiostatic gauge,

connected with it, at the constant potential V, as

shown by the gauge.

Fig. 54—The Replenisher.

ELECTROMETERS. 165

The second part of the electrometer consists of the

apparatus for expressing differences of potential, be-

tween conductors, in absolute measure. The metal

plate A, called the guard plate, has, at its center, a

circular opening about If inches in diameter, to which

is fitted the disc C; which just fills it without touching

the edges; and is made of thin aluminium, flat and

smooth on its under side, but strengthened by a rim,

and radial arms, on its upper side. It is supported by

three light steel springs, shaped somewhat like tuning-

forks, and placed horizontally, at equal distances apart;

one of which is shown at S. The lower end of each is

•attached to the center of (7, and the upper end to a

brass socket, which is cemented to the lower end of a

glass rod, shown at I; which insulates it from the

metal rod above; to the lower end of which the glass

rod is attached. And the metal rod is moved vertically

in guides by the micrometer screw M; the movements

being registered by the scale Gr, and the graduated disc

D.

To the center of the disc is attached a fine hair:

in front of which a lens, IT, is so placed as to form, at

its conjugate focus, near the surface of the jar, an

image of the hair; which may be viewed through the

eye-piece at L. This image is seen exactly betweenthe points of two screws K

ywhen the lower surfaces

of the disc C, and guard plate A, are in the same plane :

which is called the sighted position.

On a support below J., is the metal disc B, knownas the attracting disc ; insulated from the jar, and mov-able vertically by the micrometer screw M'; the move-ments being registered by the scale E, and the grad-

uated disc T. It is connected with the electrode N, by


which it can be put in electric connection with bodies

whose potential is to be tested.

The attracted plates P and C are really movable

centers of the guard plates Gr and A ; and since loss of

charge, from radiation and otherwise, affects chiefly the

outer edges, the small centers are practically unaffected

by such loss. Hence the large discs Gr and A are

appropriately called guard plates.

Mode of Using the Absolute Electeometee.—The plates are first brought to zero potential, by put-

ting them, for an instant, in electric connection, by the

electrode JV", connecting with J9, and a wire connecting

with A through the cover. The disc is then brought to

its sighted position by the micrometer 31", and the read-

ing noted. A known weight, w, is then placed upon it

so as to depress it below the level of the guard plate

A; and M is turned till is again raised to its sighted

position : the reading is noted, and the weight removed.

The Leyden jar is then charged to potential V, as

determined by the idiostatic gauge, and kept constant

by the replenishes during the experiment. The disc

B is now put into connection with the outside coating

by the electrode N; and the micrometer Mf turned till

the attraction of B on the disc C brings it again to its

sighted position. Hence the attraction of B is known to

be equal to the weight w. This reading being noted, Bis insulated, and the bodies, the difference of whose po-

tentials x and z is required, are successively put into

contact with B through A7". The distances d and h

through which B has to be moved to bring the disc (7,

in each case, to its sighted position, are noted, and the

difference of potential can then be calculated.

Formulae foe Charged Surfaces. — With a

ELECTROMETERS. 167

given charge, the electric energy at any point on a con-

ductor, called its surface density, is in proportion to its

surface area. Let q represent the surface density, then

the electric force, exerted by a charged conductor on a

point near it, equals q multiplied by the surface area.

On a sphere the surface equals the square of its ra-

dius multiplied by 4x3.14159. If 3.14159= *, and

radius = 1, we have I2 x 4 it = 4 n. Hence the force

exerted by a charged sphere on a point near it equals

4 n q ; and the force exerted by a charged hemispher-

ical surface equals 2 n q.

The hemispherical surface may be considered as

made up of the bases of an infinite number of small

cones, having their apexes at the center. Hence each

base subtends a solid angle : and lines of force, extend-

ing from surface to center, are everywhere normal to

the surface.

Now if we conceive a plane surface applied to the

hemispherical surface, and these cones extended to

meet it ; we find that the lines of force, extending from

these bases, are oblique to the plane surface. Hence

each one can be resolved into, two components, one

normal to the plane, and the other acting along it.

But since there are an infinite number of these cones,

the lines of force from whose bases may all be resolved

in this way; the components along the plane, all

around, neutralize each other, leaving only the normal

components; whose force equals the sum of all the

solid angles multiplied by the surface density, which, as

we have seen, equals 2 no. Hence the expression is

the same for a plane or a hemispherical surface.

Application of Formulae to Measurementsby Electrometer.—When there are two discs, at


different potentials, near each other, as A and B in the

electrometer, the attraction of each for the other is

equal; the air being the dielectric between them.

Hence the force, exerted at any point between them,

equals the force on both surfaces, represented b)r 4?r(>;

and tends to draw the movable disc C towards B.

But this force is also equal to the difference of poten-

tial, divided by the distance between the discs. Hence

when x represents difference of potential, and d the/*»

distance, the resultant force, at any point, equals —

.

xxHence 4c7tp=— , and Q= -

t

—;.a \7ta

Now if the surface of the movable disc C be repre-

sented by 8, its attractive force will equal s q : hence

the total attractive force equals 27tQXSQ = 27ts()2.

xAnd substituting for q its value, A

'Y , we have° 4 Tt a

2 7ts( _) = 2ns » ™= q—

r

2-

V4 rt d' 16 7i" d" 8 *r cr

Now since the attractive force equals the weight w,

multiplied by the acceleration produced by gravity,

represented by g, we have w g = ^—— : therefore x8 7t d2

— d — w 9(1), which expresses x in absolute meas-

ure. But x represents the potential of the first body

tested by the electrometer.

By a similar process the potential, z, of the second

body is expressed by the equation, z=h\ (2).

Subtracting (2) from (1), we have x— z = (d—K)

\%7twg

ELECTROMETERS. 169

By substituting figures for the letters in the second

member of this equation, the difference of potential, of

any two bodies we wish to test, may be expressed

arithmetically. ,~——

—

8 it w q .

The expression J is constant ; since it rep-

resents the attraction of the disc B for C, when the

Ley den jar is at the constant potential, V : while the

expression (d— K) is variable; representing the differ-

ence of distance, required by the variable difference of

potential, expressed by x— z.

Thomson's Quadrant Electbcmeter.—This in-

strument, invented by Sir William Thomson, is highly

esteemed for its great sensitiveness. It is represented

by Fig. 55, and consists of a frame supporting a Leyden

jar, which resembles an inverted glass shade, with a

brass cover, to which the principal parts are attached.

These consist of the idiostatic gauge and replenisher,

already described, and the quadrants and needle, and

parts connected with them.

The jar contains strong sulphuric acid : which forms

the inner coating, keeps the interior free from moist-

ure, and forms a perfect connection with the needle,

without friction. The outer coating consists of strips

of tin-foil, connected with the cover and supporting

frame. The upper part of the jar incloses the quad-

rants and needle;protecting the needle from currents

of air, and permitting its movements to be'seen.

Fig. 56 is an enlarged view of the needle and quad-

rants. The needle is a thin, ilat piece of aluminium,

shaped like a figure 8 ; represented by the dotted lines

in Fig. 56 ; and seen edgewise in its place at w, in Fig.

55. Through its center passes a piece of stout platinum

wire to which it is attached, and which terminates


above in a small, T-shaped piece of metal: to which

are attached, at the extremities of the cross piece, two

fibers of unspun silk ; by which the needle is suspended

Fig. 55—Thomson's Quadrant Electrometer.

from a projecting arm, supported, in the upper part of

the instrument, on a vertical glass rod. When the

needle is at rest, in the fixed position between the

quadrants, as shown in Fig. 56, the silk fibers hang

parallel to each other, and the cross piece, below, is

ELECTROMETERS. 171

then parallel to the projecting arm above. But in any

other position, each fiber is at an angle with its vertical

position, . and the needle slightly elevated : conse-

quently the force of gravity tends constantly to turn

the needle, without friction, back to its fixed position.

This mode of suspension is termed bifilar.'

A platinum weight, suspended in the sulphuric acid

by a fine platinum wire, from the lower end of the stiff

wire below the needle, keeps

the needle in position, and in

contact with the inner coat-

ing.

The wire, above and below

the needle, is inclosed in fixed

guard tubes ; the lower one

shown at iv : which screen it

from external electric influ-

ence; and furnish a connec-

tion, by which the charge is given to the inner coating.

The needle is inclosed within four brass quadrants

:

which, if joined, would form a circular box. They are

separated from each other, and from the needle, as

shown in Fig. 56 : and opposite pairs, A and A\ B and

B\ are connected by fine wires ; and all supported at

the same level ; and insulated, by glass rods attached

to the cover.

Three of them are permanently attached, but the

fourth can be moved in and out horizontally;guides,

and a spring and counteracting screw, being arranged

to keep it in position, and regulate its movement.

Above the needle, and attached to its supporting

wire, is a small concave mirror t ; by which a ray of

light is reflected on a scale, placed in front of it, at a

Fig. 56—Quadrants and Needle.


distance of about 36 inches. This scale is shown in

Fig. 57. Behind it is a lamp, the light from which

conies through a vertical slit in a screen : above which

is a horizontal screen, which cuts off the direct rays

from below ; while the angle of reflection brings the

ray from the mirror directly on the scale, where it

appears as a small spot of light. Another screen,

placed at an angle,

cuts off the direct

raj's from above.

As the mirror turns

with the needle, the

reflected ray becomes

a long pointer ; mov-

ing without friction :

by which the slight-

est movement of the

needle is indicated on

the scale.

In the instruments

first constructed, the needle was suspended by a single

fiber of silk ; and a small magnet attached to the backof the mirror: which, by the attraction between it anda large magnet, placed outside the jar, as shown in Fig.

55, controlled and limited the movements of the needle ;

the attraction of the magnets tending constantly to

bring it back to its fixed position, where the spot of

light rests on the zero of the scale. But the bifilar sus-

pension is now preferred ; rendering the use of magnets

unnecessary.

At I and ?n, Fig. 55, are seen the chief electrodes;

used to connect the opposite pairs of quadrants with

bodies whose potential is to be tested : and at p is the

Fig. 57—Scale, Lamp, and Screen.

ELECTROMETERS. 173

charging electrode, used to connect the replenisher with

the inner coating of the Leyclen jar. One pair of quad-

rants, A Af, Fig. 56, is connected with the electrode ?,

and the other pair, B B\ with the electrode m.

Mode of Usixg the Quadraxt Electrometer.

—The Leyden jar is connected with the replenisher by

the electrode p, and charged to a certain constant po-

tential, F, as indicated by the gauge ; and its constancy

maintained during the experiment: and the needle,

being connected with its inner coating, has therefore

the same constant potential V.

By means of the electrodes I and m, a connection is

then made between the opposite pairs of quadrants, and

any two bodies whose difference of potential is required;

one of which is usually the earth. Suppose the earth

connection to be made with the electrode m ; then, if

the potential of the other body is higher than that of

the earth, the needle will move round from the higher

to the lower potential ; that is, from A Arto B Bf

: but

if it is lower, the movement will be from B Brto A A!:

and the difference of potential will be indicated on the

scale by the movement of the spot of light, to the right

or left from zero ; and may be considered practically

correct, within certain limits. In this way the re-

quired potentials are compared with the constant poten-

tial V; and the results determined in absolute measure.

In the Helmholtz quadrant electrometer the quad-

rants are maintained at the constant potential; and the

bodies whose potential is required are connected with

the needle.

There are various styles of Thomson's electrometers

:

both of the attracted-disc and quadrant instruments.

Some of them are portable, and much simpler than


those already described; the replenisher, gauge, and

Leyden jar, being omitted; also the bifilar attachment

in the quadrant instrument; the movements of the

needle being controlled by the torsion of a fine wire.

And, in the attracted-disc electrometer, the position of

the discs is sometimes reversed; the attracting disc

being placed above, in the portable style.

CHAPTER XII.

The Electricity of the Earth and Atmosphere.

Potential and Earth Currents.

Terrestrial and atmospheric electricity are so in-

timately related, that to obtain a correct knowledge of

either requires the consideration of its relations to the

other.

Viewing electricity as a universal property of mat-

ter, its existence in the earth and atmosphere follows

as a necessary consequence. Hence, we are to study

its phenomena in this connection, rather than to ac-

count for its origin. These phenomena pertain chiefly

to difference of potential between different parts of the

earth's surface; different parts of the atmosphere; and

between the earth's surface and the atmosphere.

This difference of potential results from various

causes. We have already seen how difference of po-

tential may be produced, artificially, bj^ various instru-

ments, which are combinations of different substances,

having different degrees of electric resistance and con-

ductivity. By similar methods nature, on a grand

scale, produces results of which ours are but feeble im-

itations.

Illustrations from the Thermopile.— In the

thermopile we have an illustration of the method bywhich difference of potential is produced by heat. This


instrument is a combination of metal bars, whose con-

ductivity for heat and electricity varies greatly. Anumber of these bars, arranged in compact form, and

proper]}7- insulated, are soldered together in an alter-

nating series: so that a current of electricity, passing

through them, has to pass from one metal to the other.

They are folded together, and mounted in such a man-

ner, that heat may be applied to one set of junctions;

while the opposite, alternate set, is cooled.

In this waj^, instruments are constructed, in which a

very slight difference of temperature, between the op-

posite sets of junctions, creates a perceptible difference

of electric potential: and powerful batteries are con-

structed in the same manner.

The earth may be regarded as an immense battery of

this kind; being composed of heterogeneous materials,

whose conductivity for heat and electricity varies

greatly: raid which are subjected to great extremes of

temperature, at opposite junctions, fulfilling exactly

the conditions of the thermopile.

The ocean, a vast, homogeneous conductor, is sepa-

rated into different parts by the great continents; whose

conductivity differs from it greatly: the five great

divisions of the ocean, and the two continents, con-

stituting an alternating series of conductors, of differ-

ent conductivities.

The surface of the continents, composed of rock and

soil, of lakes, rivers, and sandy deserts, presents a great

diversity of material, of widely different conductivity.

In the torrid and frigid zones, we have the opposite

extremes of temperature; which, in the thermo-electric

battery, are produced by exposing one set of junctions

to the heat of a lamp furnace ; while the opposite set is

POTENTIAL AND EARTH CURRENTS. 177

cooled with ice. Similarly also in the diurnal revolu-

tion of the earth, opposite sides are subjected daily to a

constantly changing temperature. And, in its annual

revolution, we have the same result in the changing sea-

sons; which also produce great changes in the conduct-

ing character of the surface ; from the frozen, snow-

clad surface of winter, to the verdure-clacl surface of

summer.

Diurnal and Seasonal Variation.—The change

of electric potential produced by these causes in the

earth, induces the opposite potential in the atmosphere

;

which, by its lower strata, is insulated from it. Hence,

in observations made on the potential of the earth and

atmosphere, we find, as we should be led to expect,

daily maxima and minima potential, and also seasonal

maxima and minima.

In several series of observations, made by different

observers in Europe, both on the continent and in the

British Isles, these maxima and minima were carefully

noted: and it was found, that, in winter, the daily

maxima occur at about 10 A. m. and 7 P. M.; in sum-

mer at about 8 A. m. and 10 P. m.; and in spring and

autumn, at about 9 A. M. and 9 P. M. The daily min-

ima occur, in summer, at about 3 P.M., and midnight;

but the daily winter minima are not given with suf-

ficient definiteness to be reliable.

From this we see, that the daily maxima, occurring

soon after sunrise and sunset, correspond to the hours

of greatest change of temperature ; while the daily

minima occur at the hours when temperature is most

constant.

The seasonal maximum occurs in winter, and the

seasonal minimum in summer: the maximum about12


January, and the minimum in May and June. Theyare doubtless due, in part, to the different conduct-

ivity of the earth's surface in summer and winter,

as already mentioned ; and also to the dry winter

atmosphere, when atmospheric insulation is high, as

compared with the damp atmosphere of spring and

early summer, when it is low; the greatest minimumoccurring in the months when our atmosphere, in the

north temperate zone, is most heavily laden with vapor.

At this season the earth is covered with green, suc-

culent herbage ; wet with frequent showers, and laden

at night with heavy dews ; forming a conducting sur-

face, which offers but slight resistance to electric trans-

mission.

Towards the close of summer, the grain ripens, the

showers become less frequent, the dews lighter, and a

vast expanse of dry straw and stubble, with a parched

soil beneath it, offering high electric resistance, takes

the place of the former conducting surface. As fall ad-

vances, and the grass becomes dry and withered, and

the trees shed their leaves, there is a constant increase

of this surface resistance, and a corresponding increase

of electric potential, till the winter maximum is reached.

While this difference of conductivity in the land

surface is taking place, the conductivity of the water

surface remains practically constant : hence the period

of minimum potential corresponds to that in which the

difference of conductivity, between the land and water

surfaces, is least; while the period of maximum poten-

tial corresponds to that in which it is greatest ; point-

ing clearly to this difference as a probable cause.

In addition to the changes of electric potential, in-

duced in the atmosphere by these changes in the elec-


trie condition of the earth's surface, its electricity is

doubtless affected, directly, by conditions similar to

those which affect the earth's electricity.

In its combination of dry air and watery vapor ; the

one, an insulator, and the other, a conductor ; separate

parts heated and cooled alternately, twice in twenty-

four hours, we have thermo-electric conditions similar to

those already noticed in the earth's surface ; though

the resulting electric disturbance is, perhaps, less in-

tense, as the composition of the atmosphere is nearly

uniform, while that of the earth's surface presents great

diversity.

Difference of Potential between Atmos-

pheric Strata.—Another cause of atmospheric elec-

tric disturbance is found in the great difference of

electric resistance between the upper and lower atmos-

pheric strata; caused by the density below and rarity

above. This resistance makes the dense lower stratum,

where most of our observations are made, an excellent

insulator ; while the rarity of higher strata facilitates

electric transmission; a constant decrease of resistance

taking place, from the lower to the higher, till a point

is reached, where it is reduced to that of the ordinary

Geissler tube ; while, in still higher strata, the resist-

ance increases, on account of the extreme rarity of the

air: which equals, and finally exceeds, that of the best

vacuum tubes.

The existence of a corresponding difference of elec-

tric potential has been proved by numerous experi-

ments ; among which may be noted the following:

—

From an elevated position, a metal -pointed arrowwas shot upward to a vertical height of 250 feet: a con-

ducting cord, connected with it, and properly insulated,


communicated with an electroscope at its lower extrem-

ity. As the arrow rose, the electroscope showed a

steadily increasing difference of potential, till the

indications equaled the full capacity of the instrument.

The arrow was then shot horizontally, at an eleva-

tion of about three feet, but no change of potential was

indicated; proving' that the indications resulted from a

difference of potential existing in the atmosphere, and

were not due to the friction of the arrow in passing

through the air.

The difference of potential, in this experiment, was

between the earth and atmosphere : but the following

experiment was entirely independent of the earth.

During a balloon ascent, a conductor, 170 feet in length,

was lowered into the air ; a ball being attached to its

lower end, and its upper end connected with an elec-

troscope. The indications showed a marked difference

of potential between the upper and lower strata.

As the balloon moved with the wind, the friction

between the ball and the air could not have been suf-

ficient to affect the electroscope perceptibly ; so that,

in this instance, as in the former, the indications of the

instrument must be attributed to a difference of po-

tential existing in the atmosphere.

The series of observations already referred to, and

numerous others of a similar character, prove that the

potential of the atmosphere is almost invariably positive

with reference to that of the earth.

The Atmosphere as a Leydex Jar.—It is evi-

dent that we have, in the atmosphere and on the earth's

surface, the same conditions which exist in the Leyden

jar—two conducting surfaces insulated by a dielectric

;

the stratum of least resistance forming the upper con-


ducting surface; the earth's surface, the lower one; and

the dense lower stratum, the dielectric. And, as in

the Leyden jar, any change of potential in either sur-

face produces the opposite electric condition in the

other surface.

The upper surface, being insulated, corresponds to

the inner coating; and the lower uninsulated surface,

to the outer coating. But since those surfaces are of

vast extent, any limited area of upper surface would be

connected with a conducting surface at its outer edges;

through which connection electricity would be repelled

from this area, or attracted to it, as the potential of the

surface below it had a greater or less intensity. But

the earth connection, of the lower surface, would be

exactly the same as that* of the outer coating of the

Leyden jar.

We live and move on the outer coating of this Ley-

den jar; on a surface practically equipotential within

limited areas ; and hence do not perceive electric action

taking place, no matter how highly charged the jar

may be, except when the tension becomes strong

enough to overcome the resistance of the dielectric, or

to render prominent or visible the action on either

side of it.

This surface then, which we call neutral, is really a

charged surface ; but, like the outer coating of a

charged Leyden jar, quiescent, till brought into action

by connection with the inner coating, or by induction

between the two.

ASCENDING AND DESCENDING CUEUEXTS.—Wehave seen how air currents are produced by the action

of an electric machine, and how light bodies vibrate

between electrodes connected with opposite surfaces of


a charged Leyden jar. Now since a constant difference

of potential is proved to exist between the earth's

surface and the atmosphere, and between upper and

lower atmospheric strata, we must conclude that

ascending and descending currents result from this

difference : and that the clouds, and the invisible vapor

diffused through the air, are, like the air, subject to this

constant electric movement. But, there being also a

horizontal movement, due to the winds, the resultant

of the two movements is a series of curves, ascending

and descending, as the body of air and vapor moves

over areas of high or low potential.

The air and vapor in contact with the earth, becom-

ing electrified to the same potential as the earth's sur-

face, are repelled, and attracted upward by the force

resulting from difference of potential in the stratum of

least resistance above. Similarly the air and vapor

above are repelled, and attracted downward in conse-

quence of the difference of potential below.

The morning and evening maxima, occurring at

opposite points in the rational horizon, show that two

electric waves traverse the surface daily from east to

west, as the earth revolves from west to east. And, at

points about equally distant from these waves, follow

the two daily minima. During the maxima the ascend-

ing and descending currents must acquire a great

increase, both in volume and in acceleration of move-

ment: while the minima, preceding and following,

create horizontal movements between the areas of high

and low potential;producing resultant curves, similar

to those due to the winds, but recurring in regular

succession. In fact these currents are themselves

electric winds.


The rarefying of the air from heat, at the time of the

morning maximum, must increase and accelerate the as-

cending current, while its condensation from cold, at the

evening maximum, similarly affects the descending cur-

rent; gravity in each case supplementing electric force.

Cosmic Electric Influence. — Assuming that

electricity is a universal force, acting through matter

in different forms, as a universal medium, it follows

that electric induction is universal. Hence induction

between our planet and the other members of the solar

system, especially the sun and moon, must affect the

electric condition of the earth and atmosphere.

It is considered a well established principle, that the

tides are due to the attraction of the sun and moon,

attributed to gravity. But the daily electric maximaand minima indicate that there are electric tides, coin-

cident with the ocean tides, due to the electric induc-

tion of the sun and moon : that an electric impulse

follows the earth's movement, as different portions of

its surface are successively exposed to this influence

during its daily rotation, producing electric currents

in both the land and water surface ; and perhaps also

tidal waves in the ocean and atmosphere.

We have seen that when a charged sphere is placed

near the end of a cylinder, or of the longer axis of a

spheroid, the electricity of the cylinder or spheroid is

either repelled or attracted by induction, according as

the potential of the sphere is positive or negative, writh

reference to that of the other body ; and that this effect

is intensified when two charged spheres, at different

potentials, are placed at opposite ends of the cylinder,

or longer axis of the spheroid. If both are placed at

the same end, the inductive effect is a mean between


the two effects. But if one be placed opposite the

center of the c}~linder or spheroid, so that its action is

at right angles to that of the other, the intensity of

action at the ends is diminished.

In the sun, moon, and earth, these conditions are

exactly fulfilled as to shape and position ; and, prob-

ably also, as to difference of potential. The earth is

an oblate spheroid, whose longer axis lies east and west

;

pointing nearly to the apparent path of the sun and

moon. Hence at the full moon, the new moon, and

the quarters, we must have the same inductive effects

as in the experiment with the spheroid and the two

spheres. The earth, at full moon, is between the sun

and moon, and receives the highest inductive effect.

At new moon they are on the same side of it, and

nearly in line, and their effect, if at different potentials,

is lessened : while, at the quarters, when the induction

from each is at right angles to that of the other, it is at

its minimum. Hence we should expect to find, as in the

ocean tides, electric neap and spring, ebb and flood tides.

Very little is known of the relative inductive influ-

ence of the sun and moon on the earth. Judging from

the analogy of the ocean tides, we might infer that the

induction of the moon is greatly in excess of that of

the sun. But in estimating effects produced by gravity,

the two principal factors are mass and square of dis-

tance ; whereas, in estimating inductive electric effects,

the various agencies by which electricity is generated

must also be taken into account.

The nearness of the moon to the earth causes its

effect on the ocean tides to be much greater than that

of the sun, though its mass, as compared with the mass

of the sun, is only as 1 to 26,400,000. But, in consid-


ering the electric influence of the two bodies, we find

that the lunar surface is that of a dead world, abso-

lutely quiescent, so far as we know; while the solar

surface, to a great depth, is in a state of the most

violent agitation. From which we must infer a great

difference of electric potential in favor of the sun.

And observation indicates that this state of agitation

affects the earth's electricity ; while we have no obser-

vations of electric effects produced by the moon.

Certain electric phenomena on the earth are found

to coincide with certain solar phenomena. These con-

sist in violent oscillations of the magnetic needle

during prominent solar disturbances, indicated by the

sun spots. And it is found that the periods of max-

imum solar disturbance, which occur once in eleven

years, are noted for corresponding maxima in those

perturbations of the magnetic needle.

To make this clear, it should be stated that magnet-

ism is produced, artificially, by the circulation of an

electric current around a conductor capable of being

magnetized, at right angles to its length ; as by a

current circulating in a coil of wrire, round a bar of

iron or steel. And, conversely, a magnet generates an

electric current in such a coil.

It is known that the earth is a great natural magnet;

having north and south magnetic poles, which exercise

a directive force on the magnetic needle ; and it seems

highly probable, that its magnetism is the result of

electric waves, or impulses, circulating round it from

east to west as has been shown; giving rise to electric

currents ; and due to difference of temperature, and to

solar and lunar influences : and that the perturbations

of the magnetic needle, coincident with solar disturb-


ances, are the result of corresponding disturbances in

these electric movements.

Observations on Telegraph Lines.—The tel-

egraph affords special facilities for observing many of

the phenomena pertaining to terrestrial and atmospheric

electricity, by means of its long lines of nearly uniform

conductivity, insulated in the air, having earth con-

nections at points remote from each other, and extend-

ing, in the United States, chiefly, either at right angles

to the magnetic meridian, or parallel with it.

These facts have been recognized; and, within the

last five years, observations have been made, on a

limited scale, in the United States, and in Europe.

These observations have been somewhat desultory and

local ; no general, extended, well established system

having yet been instituted.

During the fall and winter of 1883-84, a series of

observations was made on a line belonging to the

Postal Telegraph Co.; extending, at first, from NewYork City to Meadville, Pa.; 509 miles by wire, 325

direct; but subsequently completed to Chicago; 1058

miles by wire, 725 direct. The observations from Oct.

18 to Nov. 20, 1883, were between New York and Mead-

ville; and the subsequent observations, which were con-

tinued during November and December, 1883, and part

of February, 1884, were between NewYork and Chicago.

The line consisted of a large copper wire having a

steel core ; thus combining conductivity and strength

;

and the object of the observations was to ascertain the

relations of the electric current to difference of temper-

ature. They were made daily, at both ends of the line,

at the hours when it was least occupied with other

business, 8 to 8.30 a.m., 5 to 5.30 and 11 to 11 30 p.m.


The line being disconnected from the batteries, and

connected with the earth at both ends, the current was

obtained from the earth alone, independent of any artifi-

cial source: and its strength and direction, as indicated by

the galvanometer, were noted, and also the temperature.

It was found, that the general direction of the cur-

rent was from a region of high to one of low temper-

ature, though frequent reversals of current were

observed. And as the east, from longer exposure to

the sun's heat, would have a higher temperature than

the west, at the time of the morning observation, the

prevailing current, at this hour, was found to be from

east to west. As these conditions of temperature

would be reversed in the evening, the observations at

that hour- showed a corresponding reversal, and a

prevailing w7est to east current. While the observa-

tions near midnight, when another reversal of temper-

ature is at hand, showed that the current then was

fluctuating and uncertain.

The deflection of the galvanometer needle varied

from to 57°; the morning average being 11. 4°, the

evening average 14.3°, the average near midnight 7.3°,

and the general average 11°. The difference of tem-

perature, between the points of observation, varied

from to 37°; the morning average being 14.5°, the

evening average 9.5°, the average near midnight 10.3°,

and the general average 11.4°.

When the earth connection was severed, at either

station, the current was reduced to a minimum ; cor-

responding to the probable leakage along the line

;

proving that it was an earth current, and not an

atmospheric current.

If a similar east and west line were extended round


the globe, we may reasonably infer that similar results

would be observed on every part of it: and hence,

that east and west currents are constantly traversing

the earth, as it revolves from west to east.

This will be more fully understood, when we con-

sider, that, during the diurnal revolution of the earth,

the sun occupies practically a fixed position with ref-

erence to it : so that from the earth's heated hemi-

sphere, electric currents are constantly flowing, from a

central point where the sun's rays are vertical, in

opposite directions, towards a point within the cooler

hemisphere, opposite to the sun.

But the diurnal revolution of the earth brings any

limited area of its surface, surrounding an observer,

alternately into each of these currents. So that, while

they have a fixed direction with reference to the sun,

and to the earth, as a whole; they become, alternately,

east or west currents, with reference to such an area.

From noon to midnight this area would be in the

west to east current ; and, from midnight to noon, in

the east to west current; an equatorial point, on the

observer's meridian, passing the point from which the

currents diverge, at noon ; and reaching the point

towards which they converge, at midnight.

At both these hours, the temperature, at equally

distant points in the observer's latitude, reaching from

his position, east and west to the sensible horizon, is

nearly the same : and the noon and midnight minima

of electric potential are the result.

At sunset and sunrise the temperature on similar

quadrants of the observer's latitude, east and west of his

position, attains its maximum difference; and the even-

ing and morning maxima of electric potential occur.


It will be observed that while the observer's position

reaches the point of highest temperature at noon, the

point of lowest temperature is reached at sunrise. For

the heating of any given area begins at sunrise,

increases till noon, as the sun's rays become more

vertical; and declines from that hour till sunset, as

the rays become less vertical; while the cooling is

constant from sunset to sunrise. So that the morning

difference of temperature, between east and west

regions, is greater than the evening difference ; and we

should expect to find a corresponding increase of elec-

tric potential, at the morning maximum.

But the series of telegraphic observations given

shows the reverse; which may result from the fact that

the line on which the observations were made, has the

Atlantic ocean at its eastern terminus, and the interior

of the continent at its western. And, as change of

temperature is much slower on a water surface than on

a land surface, the difference of temperature between the

Atlantic on the east, receiving the sun's rays first, and the

interior on the west, would be less in the morning than in

the evening, when these relative positions are reversed.

As the distance between heated and cooled regions

alternately increases or diminishes during the earth's

diurnal revolution, electric resistance increases or di-

minishes in the same ratio, and increase or decrease of

current intensity is a corresponding result : and electric

maxima and minima, and also reversal of current, must

follow from this cause, as well as from difference or

equality of temperature. But as increase or decrease

of distance is coincident with increase or decrease of

difference of temperature, the two causes intensify each

other's effects.

CHAPTER XIII.


The Aurora.

The relations of the aurora to terrestrial and atmos-

pheric electricity present a problem of the deepest inter-

est and importance, whose satisfactory solution must

render clear many questions now involved in doubt and

obscurity. Hence, during the last fifty years, it has

been carefully observed, and a number of important facts

in regard to it ascertained. The laws which govern it

are still far from being understood, and much con-

flict of opinion exists in regard to many points; but its

electric origin may be regarded as fully established.

This phenomenon occurs in zones surrounding the

northern and southern magnetic poles. And obser-

vations have been chiefly confined to its occurrence in

the north. The northern aurora is known as the

aurora borealis, the southern as the aurora australis,

while the term aurora polaris, or simply the aurora, is

applied to either.

In the United States it is usually first seen

at from 8 to 10 P. M., though often beginning much

later : and it continues from three to four hours. Its

occurrence during the day, also, is probable ; though

it can only be inferred from coincident effects; the

brilliancy of the daylight rendering it invisible.

THE AURORA. 191

Some of the great auroras have been seen for several

nights in succession ; their occurrence during the inter-

vening days also being highly probable.

Auroral Arches, Corona, and Streamers.—It first appears, usually, as a low arch of light, in the

direction of the pole, resembling the dawn of day;

whence its name, aurora, the morning. This arch is

often accompanied by a low bank of clouds, lying

under it, next the horizon. As the arch slowly rises

streamers of light, differing in color, size, and brilliancy,

dart up through it ; extending from the horizon to

a considerable height above the arch ; their color

varying from a pale white to a light red; though yel-

low, green, and blue tints have also been observed

;

the prevailing tints differing more or less in different

localities.

These streamers appear to radiate from a central

region below the horizon, cutting the arch vertically,

at right angles, as shown in Fig. 58. The streamers

sometimes appear to rise from widely separated points

in the horizon; and, as the aurora increases in size andbrilliancy, they culminate at the zenith, as shown in

Fig. 59, forming a corona of more or less prominence;one of the most prominent being shown in Fig. 60.

By comparing the three cuts, it will be seen, that if

the center of the corona shown in Fig. 59, or Fig. 60,

were below the horizon, the appearance would be the

same as in Fig. 58. So that, supposing the observer

placed below the horizon, under the center from whichthe streamers seem to emanate, he would see the

corona above him, as in Figs. 59 and 60. And, con-

versely, an observer in the latitude of Paris, looking at

the corona, observed in latitude 70° N., Fig. 60, would

00

i

c3

S

S3

I

co

fa

THE AURORA, 193

see only the upper part of its southern half, corre-

sponding to the aurora shown in Fig. 58.

In the aurora shown in Fig. 61, seen from the Vega,

in latitude 65° N., we have an arch formation without

streamers. A series of concentric arch segments, more

or less perfect, is seen ; the outer one less than a semi-

circle, and the most perfect of the inner ones greater

than a semicircle ; the central one, a double arch, with

the nucleus of a second double arch above the junc-

tion. From an inspection of the figure, it is evident,

that the perfect arches would appear as complete circu-

lar belts to an observer under the central point near

the horizon.

Difference of longitude, as well as latitude, must also

modify the appearance : as that portion of the arch

which appears to one observer as its summit, appears

to another, at a distant east or west point in the same

latitude, as its east or west base. And, supposing the

first observer placed in the magnetic meridian which

coincides with the center of the aurora, the effect of

perspective would cause it to assume a different appear-

ance to him, from that seen by the other observer,

viewing it from a different angle. A streamer, seen

from one position, would appear foreshortened; while

at a different angle it would appear elongated: to one

observer it might appear as a narrow ray, to another as

a broad band.

Hence, we may infer, that we see in the arch, rising

from the horizon, the outer edge of a circular belt

of electric light, with its varied phenomena of arches,

streamers, rays, and coronee, covering a large area,

parallel to the earth's surface, and extending, as it

increases in size, from a region surrounding the pole,

P4

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oc3

3

2

s

THE AURORA. 195

towards the equator : and that its different aspects, at

different times and places, and its different phases, as

seen at the same time by observers at different points,

are greatly modified by its position with reference to

the position of the observer.

Fig. 60—Auroral Corona Observed at Bossekop, Lat. 70° N.

Auroral Movement, Curtain Formation.—

A

peculiar feature of the aurora is the continual move-

ment visible in every part. A streamer darts up

rapidly from the horizon, increasing in size and brill-

ianc}^; and as rapidly fades away. Along one part of

the arch a series of streamers form in rapid succession,

giving the impression of an undulatory, horizontal

movement, at right angles to the vertical movement of

the rising streamers : and, as the intensity of this phase

decreases, a similar movement, at some distant point,

rises and declines in a similar manner. At times there

occurs a curtain formation, composed of parallel rays

;

appearing either as a single curtain, as shown in Fig.

62, or as a series of curtains, hung one behind the

other, showing only their lower margins, as in Fig. 63;

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THE AURORA. 197

undulatory movements occurring, transverse to the

apparent vertical position of the rays, like the move-

ments of a banner floating in the breeze.

This appearance is doubtless greatly modified by

perspective: the rays which are apparently vertical,

Fig. 62—Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N.

being horizontal ; and probably emanating from the

edge of an arch; producing the single curtain shown in

Fig. 62 ; or from the edges of several concentric arches,

like those shown in Fig. 61 ;producing the series of

Fig. 63—Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N.

curtains shown in Fig. 63. It is also evident from this,

that in the formation of coronae, the appearance is

probably often due to the edge of the arch, with the

streamers emanating from it, reaching the zenith of

the observer.


Auroral Bands.—Sometimes a single streamer

spans the heavens from west to east like a band. Theauthor saw such a one at Chicago, Oct. 5, 1882.

Appearing at about 10.30 p. M., near the horizon, a

little north of west, it extended, within ten minutes,

to the eastern horizon, passing near the zenith : and

remained visible for more than half an hour. Its

apparent width was about four degrees, and its color a

light red.

The signal service record, for the same date, de-

scribes an aurora, " seen generally throughout NewEngland, as far south as Washington, and, in the

northwest, from 10 30 p. M. till after midnight ; reach-

ing an altitude of 90°, and covering 90° of the horizon."

Its different colors, in different localities, were " white,

blue, yellow, and crimson. Beams, arches, waves, stream-

ers, and patches of light were visible ; and, at Wash-

ington, frequent flashes of lightning, at the edge of the

dark segment."

Height of the Aurora.—Great diversity of opin-

ion has existed in regard to the height of the aurora

above the earth. A great altitude has been assigned

to it by some, who argue that the same aurora could

not otherwise be visible to observers thousands of miles

apart : while others assign to it a low altitude ; main-

taining that these different observers do not see the same

aurora, but different ones, occurring at the same time:

since the appearance seen by one, often differs greatly

from that seen by another. But, since different parts

of the same aurora maj^ be visible to different observ-

ers, it is evident, that one of low altitude, and great

extent, might be seen at points as widely remote from

each other as the eastern and western continents ; the

THE AURORA. 199

electrified stratum of the atmosphere surrounding the

polar area, like a circular belt.

The weight of evidence is now in favor of the low

altitude ; sixty-nine miles above the surface being con-

sidered a fair estimate. But strict accuracy is not

attainable ; since it is impossible for any two observers,

at opposite ends of a base line of sufficient length, to

fix with certainty on the same point, so as to make an

angular measurement. But we can estimate the prob-

able height at which atmospheric resistance would be

sufficiently reduced to produce the auroral phenomena;

and we have already seen that this plane of least

resistance must lie between the dense strata below

and the region of high vacuum above; both of which

oppose electric movement. Hence the height, given

above, may be approximately correct ; and yet subject,

doubtless, to variation, resulting from difference of

atmospheric pressure ; low pressure diminishing resist-

ance, and depressing the auroral plane, and high pres-

sure producing the opposite effect.

Geographical Position of the Aurora.—Ob-

servation shows that the aurora is confined to com-

paratively narrow belts. It is never seen at the

equator, and is rarely visible in the northern hemi-

sphere south of latitude 40°: while in higher northern

latitudes, it is seen to the south of the observer; and

decreases in frequency and brilliancy, assuming appar-

ently a more southerly position, as the observer moves

farther north.

In Fig. 6-1, we have a chart, giving the results of

observations made in the northern hemisphere, by dif-

ferent European observers; which shows that this

auroral belt is about 30° in width. Its southern limit,


Fig. 64—Chart showing Isochasmen or Lines of Equal Auroral Frequency.

(From Petermann's Mittheilungen, 20 Band, 1874—IX.)

THE AURORA. 201

in the western hemisphere, is shown at Lat. 22° N., Long.

75° W. from Greenwich; and its northern limit, on

the same meridian, at Lat. 58° N. In the eastern

hemisphere, its southern and northern limits, on the

same meridian, are between 47° N. and 77° N.

The increased width and number of the lines, towards

the northern limit, show a great increase in the fre-

quency, brilliancy, and duration of the auroras in that

region.

It is also found, that the position of this auroral belt

varies at different seasons of the year; reaching its

southern limit near the equinoxes, and its northern

limit near the solstices.

The results given in the above chart must be regarded

as approximate, rather than strictly accurate ; as the data

on which they are based were more or less imperfect.

Causes of the Aueora.—Having now examined

the various phases of auroral phenomena, and their

location, we are prepared to investigate more fully the

causes by which they are produced.

The earth has already been described as a thermo-

electric battery, and the atmosphere as a Leyden jar;

the one a generator and the other an accumulator; and,

in the combination of the two, we may look for the

principal cause of the aurora.

We have seen that electric movement is from higher

to lower temperature, producing earth currents on east

and west lines, governed by the earth's rotation, and

by solar and lunar influence. But the greater differ-

ence of temperature between the equatorial and polar

regions must produce north and south currents of far

greater energy than these east and west currents.

It has also been shown, that a change of potential, in


any portion of the earth's surface, must produce a

corresponding change in the stratum of least resistance,

in the atmosphere above it; and that a transfer of

electricity must occur between this electrified atmos-

pheric area, and the surrounding atmosphere, lying in

the same horizontal plane, either from it or to it, as the

earth's surface below is positive or negative.

We have, in the aurora, the exact fulfillment of all

these conditions. A high earth potential, in the polar

regions, must result from the currents flowing in from

the warm region ; and produce, by induction, a corre-

sponding negative potential in the atmosphere. And,

in the belts where these ice-bound polar regions join

the warmer region, the principal electric action must

take place; producing the auroral arches of white light :

while the electricity radiating in opposite directions,

north and south from the arch, produces the streamers,

beams, rays, bands, and coronae ; as the electric action

at different points has greater or less intensity, or meets

with varying resistance.

Confirmatory evidence of this view is found in the

fact, shown by the chart on page 200, that within the

torrid and north frigid zones, where a comparatively

even temperature exists, the aurora is not seen ; and

also in the shifting position of the auroral belt with

change of temperature, as already mentioned.

The east and west earth currents must also exercise

their inductive influence, giving rise, probably, to the

transverse undulations observed in the streamers and

curtain formations. And the resultants of these cur-

rents, and the north and south currents, are seen in the

bands and streamers which often assume a diagonal

direction, northwest and southeast, or otherwise.

THE AURORA. 203

The stratum, in which these phenomena occur, must

have a certain degree of thickness; its upper surface

merging into the region of high vacuum, and its lower

surface into that of greater density ; resistance increas-

ing upwards and downwards from a central plane.

Hence, different phases of electric action must occur at

different altitudes; corresponding to the different aspects

of electric transmission in high and low vacua, seen in

laboratory experiments, as described in Chapter X

:

which may account for the common auroral appearance,

shown in Fig. 58, where the arch seems to form a back-

ground for the streamers. And, .as there is often a

series of concentric arches, as shown in Fig. 61, it is

easy to see how streamers might radiate from one arch,

across the plane of another arch, at a different altitude.

And, if one was below, and the other above the horizon,

the appearance would be the same as in Fig. 58.

Now, since the causes here assigned are in constant

operation, we may infer that there should be a constant

aurora : though it does not follow, that it should be

everywhere constantly visible. And from the great

number of auroras observed in the course of the year,

in different parts of the auroral belts, especially in the

northern part of the northern belt, it is reasonable to

infer, that, with a more perfect system of observation,

auroras, of greater or less magnitude, would be seen, at

one or more points, every night in the year.

It is also probable that this electric action may be

constant, without being always sufficiently intense to

attract attention : and that the aurora is the result of

its increased intensity.

Other atmospheric phenomena, not usually recognized

as belonging to the aurora, may also be due to this


electric action. The peculiar band and arch formation

of cirro-stratus clouds often strongly resembling auroral

bands and arches, has, by many observers, been attrib-

uted to similar electric action ; though doubtless occur-

ring at a much lower altitude than that of the aurora.

The existence of strong earth currents during the

prevalence of auroras, and of those violent perturba-

tions, known as "electric storms," are well established

facts, proved by observations on telegraph lines. Dur-

ing the aurora of Feb. 4, 1872, visible over an area

embracing 30° of latitude, and 150° of longitude, these

currents and perturbations were observed on all the

lines within this area, both land and submarine ; being

strongest on those having a southeast and northwest

direction.

The following description of the auroral storm of

Nov. 17, 1882, is condensed from the Signal Service

Reports :" Beginning a little before daylight, it was

known at first by its interference with telegraphy.

For three hours not a wire of the Western Union Tel-

egraph Company could be worked. Late in the after-

noon, the trouble seemed to decrease ; and, at night,

there was a brilliant aurora prevailing over the eastern

half of North America, the Atlantic, and northwestern

Europe: and all telegraphic service was interrupted.

Cables to Europe, and wTires to Chicago, could not be

worked ; annunciators in telephone offices dropped

;

the switch-board in Albany, N. Y., was ignited; the

switch-board and wires at Chicago were burned ; and an

incandescent lamp was illuminated at St. Paul, Minn.

A message was sent from Bangor, Me., to North Sid-

ney, C. B., 710 miles, by the earth current alone, with-

out the batteries ; the current being as strong as that

THE AURORA. 205

from 100 cells. And the short line from Boston to

Declham, ten miles, showed the disturbing influence as

much as the longer lines."

In these observations, as in those cited in Chapter XII,

it has been found that whenever the earth connection is

severed, at either end of the line, the current immedi-

ately ceases ;proving it to be an earth current, and

not a current in the atmosphere.

The increased intensity of current, on lines having a

southeast and northwest direction, noticed during the

aurora of Feb. 4, 1872, is confirmatory evidence of the

existence of resultant currents, as explained on page

202.

The hours at which maximum and minimum effects

were observed, during the aurora of Nov. 17, 1882,

correspond exactly to the hours of maxima and minima

potential, and current intensity, already cited. A max-

imum having occurred during the three morning hours,

beginning just before daylight; a minimum late in the

afternoon, and a maximum again after sunset.

Another cause of the aurora is found in the move-

ment of warm air from the torrid to the frigid zones,

and of cold air, at a lower altitude, from the frigid zones

to the torrid. The meeting and intermingling of these

opposite currents, at different temperatures, must give

rise to strong electric action in the atmosphere, similar

to that already described as taking place in the earth,

and coincident with it. And this action must occur in

the stratum next the earth, far below that assigned to

the aurora; its intensity increasing with the density of

the atmosphere, and hence being greatest at the earth's

surface.

This becomes evident, when we consider, that the


greater part of the mass of the atmosphere lies near the

earth's surface ; being included, probably, within the

first nine miles ; while the auroral stratum is supposed

to have an altitude of sixty-nine miles. Hence this

atmospheric electric action would be supplementary to

that of the earth, already described; and would have

an east and west as well as a north and south direction,

as described on page 182.

The influence of the sun and moon, already referred

to, must intensify the effects produced by other causes

:

so that we should expect to find maximum and min-

imum auroral effects, corresponding to an increase or

decrease of intensity, in solar or lunar influence. Ob-servation has shown, that such an auroral maximumoccurs, during the recurrence, once in eleven years, of

the period of the maximum solar disturbance; that

auroras are then more frequent and brilliant than at

other times : and we may reasonably infer, that future

observation will show the existence of electric maximaand minima, analogous to the tides, and auroral effects

corresponding to them.

CHAPTER XIV.


Lightning and Thunder.

Formation of Thunder Clouds.—Our investiga-

tion of this subject thus far has been confined chiefly

to the electricity of the earth and its inductive effect

on the atmosphere ; we are now to investigate the elec-

tricity of the atmosphere and its inductive effect on the

earth.

We have seen, in the Topler machine, how electric-

ity is generated by the mutual friction and induction of

insulated conductors, put in motion by mechanical force;

and collected in accumulators which acquire different

potentials, and between which a discharge finally takes

place, attended with a flash and report. Something

analogous to this occurs in the atmosphere. The clouds

are large conductors, insulated in the air, moved by the

winds, acting inductively on each other and on the

earth, and, in other respects, fulfilling the same condi-

tions found in the machine.

As the vapor forming these clouds rises from the earth,

it must have, when generated, the same electric potential

as that part of the earth from which it rises, and hencethe same difference of potential which has been shownto exist in different parts of the earth's surface.

The air laden with this rising vapor, moving along in


currents, and brought into contact with elevated parts of

the surface, and with trees, buildings, and other elevated

objects, must generate electricity by friction, much in the

same way as the carriers on the revolving plate of the

machine. And, as the vapor forms into clouds, they be-

come the accumulators of this electricity, in the same waythat it is accumulated by the plates and Leyden jars of

the machine. And this concentration of electricity in

the clouds raises their electric potential; and makesthem the nuclei to which the rising vapor is attracted in

consequence of its lower potential.

Each infinitesimal drop of vapor is a sphere with its

electric charge on the surface ; and as these drops

coalesce, and form larger ones in the cloud, the charge

on each new drop accumulates on the surface ; and as the

increase of volume is greatly in excess of the increase

of surface, the electric surface density must increase

in nearly the same ratio ; the volume representing

electric quantity, which is thus condensed on a reduced

surface, producing a corresponding increase of intensity.

Thus as a large body of invisible vapor forms first

into light fleecy clouds ; and these collect into denser

masses; there is a constant reduction of volume, and

increase of electric intensity; till the fully formed

thunder cloud is the result.

Discharge Between Clouds.—Two or more such

clouds, formed in different localities, often many miles

apart, and electrified in this manner, must, almost inevi-

tably, be at different electric potentials. And when car-

ried towards each other by opposite atmospheric currents,

at different altitudes, and brought within the sphere of

mutual electric influence, strong inductive effects are

produced ; their approach is accelerated by attraction,

LIGHTNING AND THUNDER. 209

and, when brought within proper distance, a discharge

takes place from the cloud of higher to that of lower

potential : just as a similar discharge takes place be-

tween the sliding electrodes of the machine : and the

result is chain lightning, of which the spark of the ma-

chine is an exact type.

The distance, through which this discharge takes

place, depends on the quantity and intensity of the

charge, and the difference of potential between the

clouds. It may be any distance, from a few yards to

several miles. Observation on discharges between

clouds overhanging fixed localities, as two mountain

peaks, shows that they are sometimes from three to five

miles or more in length.

We have seen how sparks, eight to ten inches in

length, are produced by the machine ; and have tested

their energy. If we compare such a discharge to that

produced between two clouds, whose magnitude and

potential, as compared with those of the machine, are

almost infinite, we can form some adequate conception

of the enormous energy of the lightning.

When the line of discharge is concealed by inter-

vening clouds, and we see only the illumination result-

ing from it, the phenomenon is known as sheet light-

ning. We have the same result, when the spark from

the machine, occurring in a dark room, is concealed.

Hence, we may reasonably infer, that the discharge be-

tween the clouds, like that between the electrodes of the

machine, would always present the appearance of chain

lightning, if the line of discharge were always visible.

The contorte'd and bifurcated discharges, known as

zigzag lightning, and forked lightning, like similar dis-

charges in the machine, are doubtless due to differences


of resistance in the air, to the induction of surrounding

clouds, and to the mutual repulsion of the molecules of

air and vapor within the line of discharge ; which, be-

ing electrified to the same potential, tend to separate and

form resultant lines, under the influence of forces act-

ing at right angles to each other.

Observation shows, that there is usually a succession

of discharges between the two clouds, similar to the

repeated discharges from a Holtz machine: in which,

after the initial charge, electricity is generated by in-

duction alone. This action begins when the edges

of the two clouds, at different altitudes, approach

within discharging distance, and come into vertical

line ; and the effect of induction is to accumulate

the electricity of the cloud of higher potential at

the end nearest to the other cloud, while the elec-

tricity of the latter is repelled to the remote end ;

just as a similar effect is produced by the mutual

approach of two differently charged conducting plates

or cylinders ; the difference of potential between the

adjacent parts being thus greatly increased.

The discharge produces a momentary equilibrium,

which is again disturbed by induction, as larger areas

of the two clouds approach more closely: the residual

becoming the initial for a new charge, further conden-

sation taking place, and fresh supplies of electricity flow-

ing in from the surrounding atmosphere. In this way

the series of discharges continues, till the clouds unite,

and complete equilibrium takes place.

When several such clouds, at different potentials and

different altitudes, collect in each other's vicinity;

as

is usually the case in a thunder storm of much magni-

tude ; the mutual inductive effect is greatly intensified.


Suppose three clouds, arranged in a series, end to

end, and so graduated as to potential, that the central

cloud is at a mean between the other two. Let a dis-

charge take place from the cloud of highest potential

to the central one ; a second discharge must quickly

follow, from the central cloud to the one of lowest po-

tential: since the first discharge has greatly increased

their difference of potential. This second discharge

would renew the difference of potential between the

first and central clouds, and prepare the way for another

series of similar discharges.

The most careless observer cannot fail to have noticed

such series of discharges, following each other in

rapid succession, in different parts of the sky, during a

violent thunder storm.

Observation also shows, that during a thunder show-

er, there is always an increase of rain-fall, and an en-

largement of the drops, within a few seconds after each

electric discharge ; the time being just sufficient for

the rain to descend, if it left the cloud at the momentof the discharge. From which we may infer, that con-

densation is a result of the discharge ; that, in the mo-mentary equilibrium which follows it, the small drops,

which were before kept apart by mutual repulsion,

from being highly charged and at the same potential,

now coalesce, and form the large drops ; which, beingtoo heavy to be sustained in the atmosphere, fall.

Thunder —As the spark from the machine is thetype of lightning, so the snap represents thunder;which is undoubtedly due to the same cause—the sud-den and intense vibratory motion of the air, in the line

of discharge, producing violent undulations in the sur-

rounding air. A cause which will appear sufficiently


adequate, when we consider the results which must fol-

low from the rush of the enormous energy of a thunder

cloud, along a line, perhaps five miles in length, in an

infinitesimal fraction of a second.

And here, as in the case of the spark, it is quite un-

necessary to suppose the passage of any material sub-

stance through the air, producing partial vacuum and

collapse, or the occurrence of anything in the nature of

an explosion, producing similar results. It is more in

accordance with the known laws of electric movement,

to suppose that the electric energy has used the air as

the medium in which to travel; and thus produced the

vibratory motion.

Common observation shows, that in explosions where

the expenditure of energy must often be far less than

in the electric discharge between clouds, the vacuumand collapse shatter window-glass in the vicinity; while

the heaviest thunder produces only a slight tremor in

adjacent buildings ; proving that such vacuum and col-

lapse cannot result from an electric discharge.

The succession of reports accompanied by a continu-

ous rumble, heard so frequently during a thunder storm,

has been considered, by some observers, as a series of

echoes from a single report; and by others, as a num-

ber of separate reports, from discharges occurring si-

multaneously, at different distances from the observer,

and heard in the order of their distance.

An echo requires the intervention of an extended

surface, as a Avail or its equivalent; and observation

shows, that the under surface of a dense thunder cloud

is of this character, being remarkably uniform, though

its upper surface may be quite the reverse : and it is also

evident, that this under surface, resting on tho denser


strata of air, and sustaining the weight of the mass of

air and vapor above, must have greater density than

the upper surface. Hence we may reasonably infer,

that this surface, and that of the earth below it, fulfill

the conditions necessary for a series of echoes.

The hypothesis of simultaneous discharges, at differ-

ent distances, may also be true in certain instances : as

it is quite possible that such simultaneous discharges

frequently occur. But the succession of reports, often

following each other with marked regularity, and steadi-

ly decreasing in volume and intensity, is not fully ex-

plained by this hypothesis, while it is entirely in ac-

cordance with the character of a series of echoes.

The re-adjustment of electric energy between differ-

ent parts of a large cloud, which must follow the pri-

mary discharge, gives rise to numerous minor discharges

;

whose sound, mingling with that from the larger air

waves, causes the rumble; analogous to the crackling

sound from similar minor discharges in the machine. Apremonitory rumble, from a similar cause, often precedes

the heavier discharge;just as the crackling precedes

the discharge of the machine.

If the cloud were a perfectly homogeneous conductor,

like a metal cylinder, this could not occur. But as it

is a mass of vapor, composed of drops insulated fromeach other by air spaces, each particular drop having its

own electric charge ; and different parts of the cloud

having different densities, and hence differing in con-

ductivity and resistance; and condensation, with increase

of potential, following the discharge, as already shown,such minor discharges, with the accompanying roar andrumble, are inevitable. Also the development of the

residual, after the primary discharge, which, in a large


cloud, must in itself have great energy, greatly intensi-

fies these effects.

Discharge from the Clouds to the Earth.—We have already seen that the potential of the atmos-

phere, and hence of the clouds, is almost invariably

positive with reference to that of the earth. Hence the

earth's surface under a thunder cloud, and all objects

connected with it, become negatively electrified by in-

duction, to the same degree that the cloud is positive

;

electricity, equal to the charge of the cloud, being re-

pelled from the earth's surface to its interior. A result of

this difference of potential is a strong attraction between

the earth and cloud, by which the cloud is drawn towards

the earth; and, unless its potential is reduced by discharge

into another cloud, a discharge to the earth is inevitable,

whenever, from reduction of distance, the resistance of

the air becomes less than the electric tension of the cloud.

When there are two clouds at different altitudes, and

a discharge takes place from the upper to the lower

cloud, the difference of potential between the latter and

the earth, being thus increased, the liability of a dis-

charge from it to the earth is increased in the same ratio.

If there are elevated objects, such as trees and build-

ings, on the surface below, the resistance between them

and the cloud is less than that of the surrounding flat

surface; not only on account of reduced distance, but

also on account of the points and angles which they

present. Hence, w^e find, that trees, flag-staffs, tele-

graph poles, church spires, chimneys, and projecting

corners of roofs are much more frequently struck by

lightning than flat surfaces.

Good conductors, such as tin gutters, metal cornices,

and ornamental iron work, also offer far less resistance


than imperfect conductors, like wood, brick, and stone;

both from their superior conductivity, and their projecting

edges and points ; and when connected with a building

and not connected by a metallic conductor with the earth,

greatly increase the liability of the building, both to re-

ceive the electric discharge, and to sustain injury from it,

by making the building its terminus instead of the earth.

Discharge from the Earth to the Clouds.—As already shown, the electricity of a large cloud, like

that of a cylinder, may be so distributed by the prox-

imity of one end to another cloud, at a lower potential,

or to an elevated portion of the earth's surface, that the

potential of this end shall be higher than that of the

remote end. The potential of the earth's surface, be-

neath it, must also be similarly affected by induction, in

reverse order; being negative where the cloud is positive,

and positive where the cloud is negative. If, under

these circumstances, the difference of potential between

the negative end of the cloud and the earth becomes

greater than the resistance of the air, a discharge from

the earth to the cloud must occur; the discharge in this,

as in all other cases, being from higher to lower potential.

These conditions are similar to those of the three

clouds already referred to : so that a discharge from

the positive end to another cloud, or to the earth, mayincrease the difference of potential between earth andcloud at the negative end.

The resistance of the earth, also, over such an exten-

sive area, retards the restoration of surface equilibrium

after the discharge from the positive end; and increases

the liability of the return discharge from the earth to

the cloud, in the ratio of this resistance to that of the

vapor of the cloud.


In this case, as in that of a discharge from the clouds

to the earth, elevated objects reduce the resistance, es-

pecially if they are good conductors, or furnished with

sharp angles or points; and become the electrodes

through which the discharge takes place.

Lightning Hods.—Franklin first proposed the

lightning rod. The identity of lightning and elec-

tricity, strange to say, was unknown, till, by the erec-

tion of a metal rod at his suggestion, and subsequently

by his well known kite experiment, sparks were drawn

from the cloud, Leyclen jars charged, and various similar

laboratory experiments, previously known to electric

science, performed by means of atmospheric electricity.

The first lightning rod was erected, May 10, 1752, a

month previous to the kite experiment, by M. Dalibard,

in France, according to the plan proposed by Franklin

for testing the identity of lightning and electrichty

:

and sparks similar to those from the electric machine

were drawn from it.

The identity of lightning and electricity having been

established, Franklin showed how the rod could be used

as a means of protecting buildings. The result is the

lightning rod, as we now have it, in its numerous forms.

And though ignorance, greed, and dishonesty have cast

their shadow upon it, yet thousands of well con-

structed rods, standing as the silent guardians of life

and property, sufficiently attest its value.

The proper construction of lightning rods was re-

cently investigated by a conference of leading English

scientists, specially appointed for that purpose : among

whom were several eminent electricians. And, after

three years of thorough investigation, during which

practical information was collected from all parts of


the world, a code of rules for the construction and

erection of lightning rods, or conductors, was adopted

December 14, 1881; which is substantially as follows:

—

Rules for the Construction and Erection of

Lightning Conductors.

Points and Upper Terminals.—As the point of

the upper terminal, from its peculiarly exposed position,

is liable to be fused by a heavy charge, it should not

be sharper than a cone whose height is equal to the

radius of its base. But, to secure the peculiar advan-

tages derived from sharp points, three or four such

points made of copper, each about six inches long,

should be attached to a copper ring ; which should be

screwed or soldered to the terminal, about twelve inches

below its highest point. And all points should be so

platinized, gilded, or nickel-plated, as to resist oxidation.

The number of terminals required, their height above

the building, and the number of conductors connected

with them, depends on the size and style of the build-

ing, and the conductivity of the material of which it is

constructed.

All elevated parts, such as turrets and spires, should

be protected by terminals: and especially chimneys,

whose liability to receive a discharge is greatly increased

by the heated air and soot.

Factory chimneys should have a copper band round the

top; with stout, sharp, copper points, each about twelve

inches long, projecting from it at intervals of two or three

feet, and specially guarded against oxidation. And the

conductor, attached to this band, should be attached to

all bands and metallic masses in or near the chimney.Space Protected.—No definite rule can be given


as to the space protected by a conductor ; as opinion

and practice vary in regard to it: but there is no well

authenticated instance of a building furnished with a

properly constructed conductor, having been injured

by lightning within a conical space, having the point

of the upper terminal for its apex, and the radius of

whose base equaled the height of the conductor.

Attachment to Building.—The evidence asrainst

the use of glass or other material, in order to insulate

the conductor, is overwhelming; and insulation maybe regarded as unnecessary and mischievous. Theattachment to the building should be made with metal

fastenings; which should be of the same metal as the

conductor itself, to prevent corrosion from galvanic

action. They should be of adequate strength: and

each should support its proper proportion of the weight.

They should not compress or distort the conductor; and

should allow free play for its expansion and contraction.

As far as practicable, it is desirable that conductors

be connected with extensive masses of metal belonging

to the building, both internal and external; except

soft metal pipes, which, from low conductivity for heat

and electricity, are liable to fusion. Gas-pipes, es-

pecially, should not be so connected on account of

liability to ignition of the gas by an electric spark,

resulting from fusion of the pipe, or from bad joints

:

but the inlet and outlet pipes of large gas meters should

always be electrically connected with each other, as a

protection against such accidents from the electric

resistance of joints; which is sometimes greatly in-

creased by india-rubber packing.

Church bells, inside well protected steeples, need

not be connected with the conductor.


Ornamental Iron Woek. — All vanes, finials,

ridge iron work, and similar ornamental metal work,

should be connected with the conductor: and it is not

absolutely necessary to use any other point than that

afforded by such ornamental work ; provided the con-

nection be perfect, and the mass of iron considerable.

As, however, there is risk of derangement through re-

pairs, it is safer to have an independent upper terminal.

Material for Conductor.—The best material for

a conductor is copper ; its weight not less than six

ounces per foot run ; and its conductivity not less than

ninety per cent, of that of pure copper. It may be

used either in the form of tape, or of wire cable, in

which no wire should be less than ISTo. 12 B. W. G.

Iron may be used, but its weight should not be less

than 2i pounds per foot run. And all iron conductors,

whether galvanized or not, should be painted, as a

protection against oxidation. Copper conductors may be

painted or not according to architectural requirements.

Form of Conductor.—The form of the conductor

does not seriously affect its conductivity : and great ex-

tent of surface in proportion to mass is not essential: but

sectional area of mass is highty essential,and should al ways

be sufficient to carry the heaviest charge without dan-

ger of fusion of the conductor, or division of the current.

The rod is desirable for long upper terminals, onaccount of its rigidity ; but the necessity of frequent

joints, and the difficulty of avoiding disfigurement of

the building, are serious objections to its use for the

body of the conductor.

Tubes are liable to the same objections: their larger

diameter, and the collars necessary for their joints, ren-

dering them more conspicuous and undesirable.


Twisted wire cables have the advantage of compara-

tive freedom from joints ; but their interstices afford a

lodgment for smoke, dirt, and water; especially if

small wires are used: which are also less capable of

resisting oxidation than large wires.

Tape has the special advantages of requiring but few

joints; of their being easily made, where necessary;

and of being flat and flexible, so that it can be adapted

to the outlines of a building, or countersunk in it and

painted over, so as not to be conspicuous.

Conductors should not be bent abruptly round sharp

corners : and in no case should the length of conductor

between the two points of a bend be more than one-

half greater than the straight line joining them. Whenpracticable, the conductor may pass straight through a

projection ; the hole being made large enough to allow

it to pass freely, without compression.

The reasons for these precautions are found in the

liability to discharge from a sharp angle, .or across a

short space in a bend.

Joints.—The most fruitful source of danger in con-

ductors is from bad joints. Screwed, scarfed, or riveted

joints, however well made, are certain to rust and cor-

rode in time ; introducing nodes of resistance, at which

the electric charge is liable either to fuse the conductor,

or to leave it and enter the building.

No joint is electrically perfect that is not metallically

continuous, and as absolutely free from resistance as

any other part of the conductor: and careful soldering,

in addition to the screwing, scarfing, or riveting, is the

only certain means of securing this, which has borne

the test of experience.

Earth Connection.—A good earth connection, for


the lower terminal, is of the utmost importance ; and

in a majority of cases of injury to buildings from badly

constructed conductors, such injury is traceable to

imperfect earth terminals-.

The terminal should connect with damp earth, at a

sufficient depth below the surface, to insure permanent

dampness, and hence permanent conductivity. And,

to render this connection more complete, it should

bifurcate below the surface ; and be connected by sol-

dering, with a mass of metal, buried in the earth. The

hole, in which this mass is buried, should be filled to the

surface with cinders or coke, to facilitate the percolation

of water; and any available drainage of pure water,

from rain water pipes or otherwise, connected with it.

The metal mass may be of copper or galvanized iron,

having about eighteen square feet of surface. Andwhere permanently damp earth is not available, it

should consist of three or four hundred pounds of iron.

Where the use of large iron water or gas mains is

available, a connection by a copper strip, can be madewith them ; no risk being incurred by such connection,

as in the case of internal supply pipes.

Inspect cox.—Periodical inspection, and careful elec-

tric testing, are requisite to maintain the system in

efficient order; as points may corrode or become fused,

joints become electrically imperfect, connections be-

come severed above or below ground, or other im-

perfections occur, from alterations in the building, andthe carelessness or ignorance of occupants or workmen.

The author has, on his house, a copper tape conductor,constructed in accordance with these principles, anderected twenty-three years ago ; and neither the house,


nor the conductor, has ever received the slightest injury

from lightning; while numerous instances of damageto buildings and conductors havfi occurred in the vicin-

ity. Which, considering the length of time, the

exposed position, and the repeated thunder storms of

great severity, which have occurred, is strong negative

evidence of the value of the conductor, and the correct-

ness of the rules here given.

Silext Discharge.—The protection afforded by a

lightning conductor does not consist, so much, in its

being the avenue by which a destructive discharge maypass harmlessly between the earth and cloud ; as in

preventing its occurrence, by a gradual, silent discharge

through the points of the conductor; by which the

accumulated energy is reduced, before it can acquire

sufficient tension to overcome the resistance of the air,

and produce a full, sudden, disruptive discharge.

This is strikingly illustrated by the gradual, silent

discharge of a large, powerfully charged Leyden bat-

tery, through the point of a cambric needle ; and is

confirmed by the brush discharge, often observed,

during thunder storms, on the points of lightning

conductors, and on the tips of the masts and yard-arms

of ships.

As a building must be regarded, electrically, as an

elevated part of the earth's surface, the importance of

as perfect an electric connection between it and the

conductor, as practicable, is apparent, in order to

secure the full benefit of protection in the manner de-

scribed; which is impaired by the resistance caused by

the use of insulators.

It is also apparent, that the conductor affords equal

protection whether the discharge is from the cloud to


the earth, or from the earth to the cloud ; as in either

case, the discharge will follow the path of least resist-

ance ; which is always through the conductor, when

properly constructed.

Heat Lightning.—The phenomenon, known as

heat lightning, is probably nothing more than the or-

dinary electric discharge from clouds invisible to the

observer, and so distant that the thunder is inaudible.

Such lightning is generally observed at night, near the

horizon ; and close observation will show, either the

existence of clouds, indistinctly visible in the darkness,

or the probability of the discharge occurring from

clouds below the horizon.

Its existence, independent of clouds, is claimed from

the fact, that it has been observed when no thunder

storm had occurred within a radius of one hundred

miles. But, not only lightning, but clouds are often

visible at greater distances. On the level surface

round Chicago, the author has frequently observed

heavy thunder storms, eighty miles distant, as shownby subsequent reports, when both clouds and lightning

were distinctly visible, though the thunder was not

audible.

Tornadoes.—As an electric origin has been claimed

for tornadoes, it is proper to remark, in conclusion,

that recent investigation has demonstrated that they

are chiefly due to currents of air, generated by differ-

ences of atmospheric temperature and pressure, andmodified by other causes: and while electricity mayintensify their force, it cannot be considered as their

primary cause.

224 elements of static electricity.

Note referred to ox Page 118.

The brush from K makes its appearance first, and

increases in length till the brush from V appears; after

which it decreases in the same ratio as the brush from

V increases, till the discharge occurs, when both dis-

appear. This is sufficiently explained by increase and

decrease of difference of potential at different points.

As the potential of the revolving plate A increases, the

difference of potential between the inside coating of the

jar (7, and that part of A which receives the charge

from it through the comb K, decreases, as indicated by

the decrease in brush-length, till the potential of both

is the same, when the brush disappears.

In like manner the potential of that part of the plate

A, passing the comb i, continues to increase till it

equals the potential of the inside coating of the jar D;

and this charged surface, passing on to the comb H, the

surplus of charge which D, from increase of potential

rejects, escapes through H to the comb V, and from Vto that part of the plate A between V and if, as indi-

cated by the increase of brush-length from V.

This process is greatly intensified by the inductive

effect of the high potential of the lower part of inductor

J7

, and low potential of the upper part of inductor X,

by which electricity is repelled from the corresponding

lower part of the plate A to its corresponding upper

part.

INDEX.

Absolute Electrometer, Thomson's, 161-169.

Accumulators, 72-91.

Amber, 1.

Atmosphere, the, as a Leyden jar, 180, 181.

Atmospheric potential, 177-1S0.

strata, difference of potential between,

179, 180.

currents, 181-183.

Attraction and repulsion, 1-4, 15, 40-42.

Aurora, the, 190-206.

, height of the, 198, 199.

,geographical position of the, 199-201.

, causes of the, 201-206.

, tubes, 146, 147.

Auroral arches, coronas, and streamers, 191-

195.

movement, curtain formation, 195-197.

bands, 198.

B

Bag experiment, 60.

Balanced rod, the, 2.

Bath, electric, 142, 143.

Battery, the Leyden. 79, 80.

Bells, electric, 102, 103, 125, 126.

Brush discharge, 117, 118, 134, 137.

Charge defined, 22.

.multiplication of, in Topler machine,121, 122.

, variation of, 67.

Charged surfaces, formulas for, 167.

Chime, electric, for frictional machine, 102,

103.

. for Topler machine. 125, 126.

Condensation, surface, 55-58.

Condensers, 74.

15

Conductivity for heat and electricity com-pared, 37, 38.

Conductors and non-conductors, 4-6.

, hollow, 58, 59-66.

Conservation of energy, the, 23-26.

Convection, 66, 67.

Cosmic electric influence, 183-186.

Coulomb's torsion balance, 156-161.

Currents, atmospheric, 181-183.

, earth, 1S6-189, 204, 205.

Cylinder, electrified, 48. 69.

with points, 70.

DDielectric denned, 50.

, required thickness of, 74.

Disc, electrified, 71.

Discharge, apparent time of, 126-128.

, brush, 117. 118, 134, 137.

between clouds, 20S-211.

from the clouds to the earth, 214, 215.

from the earth to the clouds, !J5, 126.

, disruptive, 88.

, silent, 89.

, spontaneous, 88.

through hook, 81-S4.

Discharger, 76.

, universal, 87, 88.

Dual theory, the, 40-42.

E

Earth currents, 1S6-189, 204, 205.

Ebonite, 1,6, 58, 54.

Electricity, the nature of, 23-42.

of the earth and atmosphere, 175-223.

generated by the friction of metals, 132.

133.

Electrics. 4.

Electric bath, 142,143.

226 INDEX.

Electric movement, 13-16.

potential, 10-11.

transmission in vacua, 148-154.

wind, 104, 105. 143.

Electrometers, 155-174.

, attracted disc, 161.

Electrometer, Thomson's absolute. 161-169.

,mode of using the absolute, 166-169.

, Thomson's quadrant, 169-174.

, mode of using the quadrant, 173, 174.

Electrophorus, the, 92-96.

Electroscope, the gold leaf, 16-18.

, the pith ball. 2 3.

, charged by induction, 44.

Energy, the conservation of, 23-26.

, radiant, 31.

Ether, 31-33.

Equipotential. 55.

Experiments with the Topler machine.125-145.

F

Farad iv's hollow cube, 65.

Faradic current. 141.

Figures, Liehtenberg's, 89-91.

Force, 1.

, lines of. 55.

Form, influence of. 67.

Formulae for charged surfaces, 167.

, application of, to measurement by elec-

trometer, 167-109.

Fracture of Leyden jar, 83, 140.

Friction, mutual effects of, 18-21.

Frictional electricity, 8, 9.

machine, 96-100.

G

Gauge, idiostatic, for electrometer, 63.

Gas lighting, 143-145.

GeissLr tubes, 147, 148.

Generators, electric, 92-124.

Glass for Leyden jars, 77.

illuminated by electricity, 151.

, required thickness of, for insulation. 74.

, specific inductive capacity of, 53, 54.

Gravity and electiicity compared, 13, 14.

Gunpowder, method of exploding by elec-

tricity, 87.

Heat and electricity compared, 13, 14, 33, 37,

38.

Heat, light, and electricity compared, 26-31.

Heat lightning, 223.

Heating effects of electricity in high vacua,153, 154.

Hollow conductors, 58-66.

Hollow cube, Faraday's. 65.

Holtz machine, the, 108-110, 122-124.

Holtz and Topler machines compared, 122-

124.

Holtz, Dr. W., correspondence with, 123, 124.

Hydro-electro machine, Armstrong's, 105-

107.

Idiostatic gauge for electrometer, 163.

Image plates, 103, 104.

Induction. 43-54.

, theory of, 4S, 49.

varies inversely as square of distance,

47.

Inductive capacity, specific. 51-54.

influence of dielectric, 49-51.

Influence machines, 108.

Insulator defined, 6.

Intensity, electric, 6-8.

Jar. the Leyden. 75-91.

Jar D,in Topler machine,higher potential of,

139, 140.

Leyden jar, the, 75-91.

, charged by cascade, 77-79.

.discharged through booVc,81-S4.

, electromotive force of, 77.

, fractured by overcharge, 88, 140.

,glass suitable for, 77.

, Lane's unit, 101, 102.

, residual charge of, 84, 85.

, spontaneous discharge of, 88.

, the atmosphere as a. 180, 181.

. with movable coatings, 85, 86.

Leyden battery, the, 79, 80.

, Tyndall's experience with, 87.

Liehtenberg's figures, 89-91.

Light, heat, and electricity compared, 2^31,

138.

,polarized and electricity, 28-31, 3(5.

Lightning and thunder. 207-223.

Lightning conductors, 216-221.

, attachment of, 218.

, earth connection of, 221.

, form of, 219, 220.

, inspection of, 221.

INDEX. 227

Lightning conductors, joints of, 220.

, material for, 219.

, points for, 217.

, silent discharge of, 222.

. space protected by, 217, 218.

, test of copper tape, 221, 222.

Lines of force, 55.

M

Machine, Armstrong's hydro-electric, 105-

107.

described by Xoad, 100.

, Motional, 96-100.

, the Holtz, 108-110.

, the Topler, 110-122.

Machines compared, Holtz and Topler, 122-

124.

, influence, los.

Measurement of energy, 100-102.

Medical treatment by electricity, 142, 143.

Metals electrified by friction, 4, 5, 132.

Metal screen, inductive action of, 152, 153.

Mode of action of the frictional machine, 99,

100.

of the Holtz machine, 122, 128.

of the Topler machine, 115-124.

Multiplication of charge in Topler machine,

121, 122.

NNature of electricity, 23-42.

Negative charge, 22.

potential, 12, 13, 21.

sign, 13.

Non-conductors, 4, 5, 6.

Non electrics, 4.

o

Ozone, generation of, 131.

Pail experiment, CO-65.

Pane, the charged, 72-74.

Plates, image, 103. 104.

Points, air current from, 104, 105.

, influence of, C9, 70.

Polarized light and electricity, 28-31, 36.

Proof plnne, 58, 59.

Positive and negative, 12, 13, 21.

sign, 13.

Potential, atmospheric, 177-180.

and earth currents, 175-1S9.

, el-ctric, 10-22,

, difference of, 11, 12.

, difference of, between atmospheric

strata, 179, 180.

, diurnal and seasonal variation of, 177-

179.

of jar D, in Topler machine. 139, 140.

, reversal of, in Topler machine, 120, 140.

, zero, 13, 65, 66.

Power, transmission of, by static electricity,

128, 129.

Quadrant electrometer, Thomson's, 169-174.

Quantity and intensity, 6-8.

RRadiant energy, 31.

matter, 154.

Replenisher for electrometer, 164.

Repulsion, 1-4, 15, 16, 159.

Residual charge, 84, 85.

Reversal of potential in Topler machine, 120,

140.

Rotation of Topler machine, direct and re-

versed, 138, 139.

Rotary movement in vacua, 149-151.

s

Silent discharge, 89.

Source of electric supply of the Topler ma-chine, 129-132.

Spark, the, its direction, subdivision, andcolor, 133-138.

, and snap, 39, 40.

Specific inductive capacity, 51-54.

Spheres, electrified, 68. C9.

Spheroid, electrified, 70, 71.

Spontaneous discharge, 88.

Static electricity defined, 8, 9.

Surface condensation, 55-58.

, thickness of electrified, 66.

transmission, 58.

Telegraph lines, observations on, 1S6-18S, 204,

206

Tides, electric, 183, 184.

228 IXDEX.

Time of electric discharge, 126-123.

Thermopile, illustrations from the, 175, 17<

Thickness of electrified surface, 66.

Thunder, 211, 214.

clouds, formation of, 207-208.

Topler machine, the, 110-122.

, the four-plate, 114.

, experiments with, 125-145.

, mode of action of, 115-124.

Tornadoes, 223.

Torsion balance, Coulomb's, 156-161.

, inaccuracy of the, 159 161.

Transmission, electric, in vacua, 146-154.

of power by static electricity, l_s, 129.

, surface, 58.

Tubes, Geissler, 147, 148.

Tube, vacuum, 146, 147.

UUniversal discharger, 87, 83.

Unit jar, Lane's, 101, 102.

Vacua, electric transmission in, 32, 146-154.

, electric transmission in low, 146-149.

, electric transmission in high, 149-154.

, rotary movement in high, 149-151.

Vacuum tube, 146, 147.

wWave theory, the, 31-37.

Whirl, the electric, 104.

Wind, electric, 104, 105, 143,

Zero potential. 13. 65, (

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